diff --git a/homeassistant/components/nightscout/__init__.py b/homeassistant/components/nightscout/__init__.py
index 69d79d1cecb5d5..68bb200504c497 100644
--- a/homeassistant/components/nightscout/__init__.py
+++ b/homeassistant/components/nightscout/__init__.py
@@ -5,14 +5,14 @@
 from py_nightscout import Api as NightscoutAPI
 
 from homeassistant.config_entries import ConfigEntry
-from homeassistant.const import CONF_API_KEY, CONF_URL
+from homeassistant.const import CONF_API_KEY, CONF_UNIT_OF_MEASUREMENT, CONF_URL
 from homeassistant.core import HomeAssistant
 from homeassistant.exceptions import ConfigEntryNotReady
 from homeassistant.helpers import device_registry as dr
 from homeassistant.helpers.aiohttp_client import async_get_clientsession
 from homeassistant.helpers.entity import SLOW_UPDATE_WARNING
 
-from .const import DOMAIN
+from .const import DOMAIN, MG_DL
 
 PLATFORMS = ["sensor"]
 _API_TIMEOUT = SLOW_UPDATE_WARNING - 1
@@ -29,6 +29,11 @@ async def async_setup_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
     except (ClientError, AsyncIOTimeoutError, OSError) as error:
         raise ConfigEntryNotReady from error
 
+    if not entry.options:
+        hass.config_entries.async_update_entry(
+            entry, options={CONF_UNIT_OF_MEASUREMENT: MG_DL}
+        )
+
     hass.data.setdefault(DOMAIN, {})
     hass.data[DOMAIN][entry.entry_id] = api
 
diff --git a/homeassistant/components/nightscout/config_flow.py b/homeassistant/components/nightscout/config_flow.py
index 1f3f62835bc338..0559d5ab814e1c 100644
--- a/homeassistant/components/nightscout/config_flow.py
+++ b/homeassistant/components/nightscout/config_flow.py
@@ -7,9 +7,10 @@
 import voluptuous as vol
 
 from homeassistant import config_entries, exceptions
-from homeassistant.const import CONF_API_KEY, CONF_URL
+from homeassistant.const import CONF_API_KEY, CONF_UNIT_OF_MEASUREMENT, CONF_URL
+from homeassistant.core import callback
 
-from .const import DOMAIN
+from .const import DOMAIN, MG_DL, MMOL_L
 from .utils import hash_from_url
 
 _LOGGER = logging.getLogger(__name__)
@@ -62,6 +63,37 @@ async def async_step_user(self, user_input=None):
             step_id="user", data_schema=DATA_SCHEMA, errors=errors
         )
 
+    @staticmethod
+    @callback
+    def async_get_options_flow(config_entry):
+        """Get the options flow for this handler."""
+        return NightscoutOptionsFlowHandler(config_entry)
+
+
+class NightscoutOptionsFlowHandler(config_entries.OptionsFlow):
+    """Handle a option flow for Nightscout."""
+
+    def __init__(self, config_entry: config_entries.ConfigEntry) -> None:
+        """Initialize options flow."""
+        self.config_entry = config_entry
+
+    async def async_step_init(self, user_input=None):
+        """Handle options flow."""
+        if user_input is not None:
+            return self.async_create_entry(title="", data=user_input)
+
+        data_schema = vol.Schema(
+            {
+                vol.Optional(
+                    CONF_UNIT_OF_MEASUREMENT,
+                    default=self.config_entry.options.get(
+                        CONF_UNIT_OF_MEASUREMENT, MG_DL
+                    ),
+                ): vol.In({MG_DL, MMOL_L}),
+            }
+        )
+        return self.async_show_form(step_id="init", data_schema=data_schema)
+
 
 class InputValidationError(exceptions.HomeAssistantError):
     """Error to indicate we cannot proceed due to invalid input."""
diff --git a/homeassistant/components/nightscout/const.py b/homeassistant/components/nightscout/const.py
index 7e47f7ff49d4a2..33944f423a8c63 100644
--- a/homeassistant/components/nightscout/const.py
+++ b/homeassistant/components/nightscout/const.py
@@ -2,6 +2,10 @@
 
 DOMAIN = "nightscout"
 
+MMOL_L = "mmol/L"
+MG_DL = "mg/dL"
+
 ATTR_DEVICE = "device"
+ATTR_SGV = "sgv"
 ATTR_DELTA = "delta"
 ATTR_DIRECTION = "direction"
diff --git a/homeassistant/components/nightscout/sensor.py b/homeassistant/components/nightscout/sensor.py
index 1b37fa8da7cadd..356ad2061a59f4 100644
--- a/homeassistant/components/nightscout/sensor.py
+++ b/homeassistant/components/nightscout/sensor.py
@@ -10,17 +10,18 @@
 
 from homeassistant.components.sensor import SensorEntity
 from homeassistant.config_entries import ConfigEntry
-from homeassistant.const import ATTR_DATE
+from homeassistant.const import ATTR_DATE, CONF_UNIT_OF_MEASUREMENT
 from homeassistant.core import HomeAssistant
 from homeassistant.helpers.entity_platform import AddEntitiesCallback
 
-from .const import ATTR_DELTA, ATTR_DEVICE, ATTR_DIRECTION, DOMAIN
+from .const import ATTR_DELTA, ATTR_DEVICE, ATTR_DIRECTION, DOMAIN, MMOL_L
 
 SCAN_INTERVAL = timedelta(minutes=1)
 
 _LOGGER = logging.getLogger(__name__)
 
 DEFAULT_NAME = "Blood Glucose"
+MMOL_CONVERSION_FACTOR = 18
 
 
 async def async_setup_entry(
@@ -30,20 +31,20 @@ async def async_setup_entry(
 ) -> None:
     """Set up the Glucose Sensor."""
     api = hass.data[DOMAIN][entry.entry_id]
-    async_add_entities([NightscoutSensor(api, "Blood Sugar", entry.unique_id)], True)
+    async_add_entities([NightscoutSensor(api, "Blood Sugar", entry)], True)
 
 
 class NightscoutSensor(SensorEntity):
     """Implementation of a Nightscout sensor."""
 
-    def __init__(self, api: NightscoutAPI, name, unique_id):
+    def __init__(self, api: NightscoutAPI, name, entry):
         """Initialize the Nightscout sensor."""
         self.api = api
-        self._unique_id = unique_id
+        self._unique_id = entry.unique_id
         self._name = name
         self._state = None
         self._attributes = None
-        self._unit_of_measurement = "mg/dL"
+        self._unit_of_measurement = entry.options[CONF_UNIT_OF_MEASUREMENT]
         self._icon = "mdi:cloud-question"
         self._available = False
 
@@ -70,7 +71,7 @@ def available(self):
     @property
     def native_value(self):
         """Return the state of the device."""
-        return self._state
+        return self._get_bgl(self._state)
 
     @property
     def icon(self):
@@ -94,7 +95,7 @@ async def async_update(self):
             self._attributes = {
                 ATTR_DEVICE: value.device,
                 ATTR_DATE: value.date,
-                ATTR_DELTA: value.delta,
+                ATTR_DELTA: self._get_bgl(value.delta),
                 ATTR_DIRECTION: value.direction,
             }
             self._state = value.sgv
@@ -103,6 +104,11 @@ async def async_update(self):
             self._available = False
             _LOGGER.warning("Empty reply found when expecting JSON data")
 
+    def _get_bgl(self, value) -> float:
+        if self._unit_of_measurement == MMOL_L:
+            return round((value / MMOL_CONVERSION_FACTOR), 1)
+        return value
+
     def _parse_icon(self) -> str:
         """Update the icon based on the direction attribute."""
         switcher = {
diff --git a/homeassistant/components/nightscout/strings.json b/homeassistant/components/nightscout/strings.json
index 709788c581830a..1390cc9699f7a1 100644
--- a/homeassistant/components/nightscout/strings.json
+++ b/homeassistant/components/nightscout/strings.json
@@ -18,5 +18,14 @@
         "abort": {
             "already_configured": "[%key:common::config_flow::abort::already_configured_device%]"
         }
+    },
+    "options": {
+      "step": {
+        "init": {
+          "data": {
+            "unit_of_measurement": "Unit of measurement"
+          }
+        }
+      }
     }
 }
diff --git a/homeassistant/components/nightscout/translations/en.json b/homeassistant/components/nightscout/translations/en.json
index d8b4c441283ec7..c7a7a1d6a00380 100644
--- a/homeassistant/components/nightscout/translations/en.json
+++ b/homeassistant/components/nightscout/translations/en.json
@@ -19,5 +19,14 @@
                 "title": "Enter your Nightscout server information."
             }
         }
+    },
+    "options": {
+      "step": {
+        "init": {
+          "data": {
+            "unit_of_measurement": "Unit of measurement"
+          }
+        }
+      }
     }
-}
\ No newline at end of file
+}
diff --git a/homeassistant/components/nightscout/translations/pt-BR.json b/homeassistant/components/nightscout/translations/pt-BR.json
index 68dc0756725ba6..c57560922cdb78 100644
--- a/homeassistant/components/nightscout/translations/pt-BR.json
+++ b/homeassistant/components/nightscout/translations/pt-BR.json
@@ -14,5 +14,14 @@
                 }
             }
         }
-    }
+    },
+    "options": {
+        "step": {
+          "init": {
+            "data": {
+              "unit_of_measurement": "Unidade de medida"
+            }
+          }
+        }
+      }
 }
\ No newline at end of file
diff --git a/homeassistant/components/nightscout/translations/pt.json b/homeassistant/components/nightscout/translations/pt.json
index 093b7775829ff3..c879e89670ce0d 100644
--- a/homeassistant/components/nightscout/translations/pt.json
+++ b/homeassistant/components/nightscout/translations/pt.json
@@ -16,5 +16,14 @@
                 }
             }
         }
-    }
+    },
+    "options": {
+        "step": {
+          "init": {
+            "data": {
+              "unit_of_measurement": "Unidade de medida"
+            }
+          }
+        }
+      }
 }
\ No newline at end of file
diff --git a/tests/components/nightscout/__init__.py b/tests/components/nightscout/__init__.py
index 6c1a34ebe41e11..ee05fd7176c016 100644
--- a/tests/components/nightscout/__init__.py
+++ b/tests/components/nightscout/__init__.py
@@ -5,8 +5,13 @@
 from aiohttp import ClientConnectionError
 from py_nightscout.models import SGV, ServerStatus
 
-from homeassistant.components.nightscout.const import DOMAIN
-from homeassistant.const import CONF_URL
+from homeassistant.components.nightscout.const import (
+    ATTR_DELTA,
+    ATTR_SGV,
+    DOMAIN,
+    MG_DL,
+)
+from homeassistant.const import CONF_UNIT_OF_MEASUREMENT, CONF_URL
 
 from tests.common import MockConfigEntry
 
@@ -17,6 +22,9 @@
         )
     )
 ]
+
+CONVERTED_MMOL_VALUES = {ATTR_SGV: 9.4, ATTR_DELTA: -0.3}
+
 SERVER_STATUS = ServerStatus.new_from_json_dict(
     json.loads(
         '{"status":"ok","name":"nightscout","version":"13.0.1","serverTime":"2020-08-05T18:14:02.032Z","serverTimeEpoch":1596651242032,"apiEnabled":true,"careportalEnabled":true,"boluscalcEnabled":true,"settings":{},"extendedSettings":{},"authorized":null}'
@@ -29,11 +37,16 @@
 )
 
 
-async def init_integration(hass) -> MockConfigEntry:
+async def init_integration(hass, unit_of_measurement=MG_DL) -> MockConfigEntry:
     """Set up the Nightscout integration in Home Assistant."""
+    options = {}
+    if unit_of_measurement:
+        options[CONF_UNIT_OF_MEASUREMENT] = unit_of_measurement
+
     entry = MockConfigEntry(
         domain=DOMAIN,
         data={CONF_URL: "https://some.url:1234"},
+        options=options,
     )
     with patch(
         "homeassistant.components.nightscout.NightscoutAPI.get_sgvs",
diff --git a/tests/components/nightscout/test_config_flow.py b/tests/components/nightscout/test_config_flow.py
index 9be3c95ef4236b..42bf299b3e03cc 100644
--- a/tests/components/nightscout/test_config_flow.py
+++ b/tests/components/nightscout/test_config_flow.py
@@ -4,9 +4,9 @@
 from aiohttp import ClientConnectionError, ClientResponseError
 
 from homeassistant import config_entries, data_entry_flow, setup
-from homeassistant.components.nightscout.const import DOMAIN
+from homeassistant.components.nightscout.const import DOMAIN, MG_DL, MMOL_L
 from homeassistant.components.nightscout.utils import hash_from_url
-from homeassistant.const import CONF_URL
+from homeassistant.const import CONF_UNIT_OF_MEASUREMENT, CONF_URL
 
 from tests.common import MockConfigEntry
 from tests.components.nightscout import (
@@ -115,6 +115,54 @@ async def test_user_form_duplicate(hass):
         assert result["reason"] == "already_configured"
 
 
+async def test_option_flow_default(hass):
+    """Test config flow options."""
+    entry = MockConfigEntry(
+        domain=DOMAIN,
+        data=CONFIG,
+        options=None,
+    )
+    entry.add_to_hass(hass)
+
+    result = await hass.config_entries.options.async_init(entry.entry_id)
+
+    assert result["type"] == data_entry_flow.RESULT_TYPE_FORM
+    assert result["step_id"] == "init"
+
+    result2 = await hass.config_entries.options.async_configure(
+        result["flow_id"],
+        user_input={},
+    )
+    assert result2["type"] == data_entry_flow.RESULT_TYPE_CREATE_ENTRY
+    assert result2["data"] == {
+        CONF_UNIT_OF_MEASUREMENT: MG_DL,
+    }
+
+
+async def test_option_flow(hass):
+    """Test config flow options."""
+    entry = MockConfigEntry(
+        domain=DOMAIN,
+        data=CONFIG,
+        options={CONF_UNIT_OF_MEASUREMENT: MG_DL},
+    )
+    entry.add_to_hass(hass)
+
+    result = await hass.config_entries.options.async_init(entry.entry_id)
+
+    assert result["type"] == data_entry_flow.RESULT_TYPE_FORM
+    assert result["step_id"] == "init"
+
+    result = await hass.config_entries.options.async_configure(
+        result["flow_id"],
+        user_input={CONF_UNIT_OF_MEASUREMENT: MMOL_L},
+    )
+    assert result["type"] == data_entry_flow.RESULT_TYPE_CREATE_ENTRY
+    assert result["data"] == {
+        CONF_UNIT_OF_MEASUREMENT: MMOL_L,
+    }
+
+
 def _patch_async_setup_entry():
     return patch(
         "homeassistant.components.nightscout.async_setup_entry",
diff --git a/tests/components/nightscout/test_sensor.py b/tests/components/nightscout/test_sensor.py
index 5e73c75d93c619..71d6bee0eb0279 100644
--- a/tests/components/nightscout/test_sensor.py
+++ b/tests/components/nightscout/test_sensor.py
@@ -4,10 +4,13 @@
     ATTR_DELTA,
     ATTR_DEVICE,
     ATTR_DIRECTION,
+    ATTR_SGV,
+    MMOL_L,
 )
 from homeassistant.const import ATTR_DATE, ATTR_ICON, STATE_UNAVAILABLE
 
 from tests.components.nightscout import (
+    CONVERTED_MMOL_VALUES,
     GLUCOSE_READINGS,
     init_integration,
     init_integration_empty_response,
@@ -25,6 +28,14 @@ async def test_sensor_state(hass):
     )
 
 
+async def test_sensor_state_options_changed(hass):
+    """Test sensor state data with options changed."""
+    await init_integration(hass, MMOL_L)
+
+    test_glucose_sensor = hass.states.get("sensor.blood_sugar")
+    assert test_glucose_sensor.state == str(CONVERTED_MMOL_VALUES[ATTR_SGV])
+
+
 async def test_sensor_error(hass):
     """Test sensor state data."""
     await init_integration_unavailable(hass)
@@ -55,3 +66,19 @@ async def test_sensor_attributes(hass):
     assert attr[ATTR_DEVICE] == reading.device  # pylint: disable=maybe-no-member
     assert attr[ATTR_DIRECTION] == reading.direction  # pylint: disable=maybe-no-member
     assert attr[ATTR_ICON] == "mdi:arrow-bottom-right"
+
+
+async def test_sensor_attributes_options_changed(hass):
+    """Test sensor attributes."""
+    await init_integration(hass, MMOL_L)
+
+    test_glucose_sensor = hass.states.get("sensor.blood_sugar")
+    reading = GLUCOSE_READINGS[0]
+    assert reading is not None
+
+    attr = test_glucose_sensor.attributes
+    assert attr[ATTR_DATE] == reading.date  # pylint: disable=maybe-no-member
+    assert attr[ATTR_DELTA] == CONVERTED_MMOL_VALUES[ATTR_DELTA]
+    assert attr[ATTR_DEVICE] == reading.device  # pylint: disable=maybe-no-member
+    assert attr[ATTR_DIRECTION] == reading.direction  # pylint: disable=maybe-no-member
+    assert attr[ATTR_ICON] == "mdi:arrow-bottom-right"