diff --git a/integrations.go b/integrations.go
index eb11aa977..1c2f5d6d5 100644
--- a/integrations.go
+++ b/integrations.go
@@ -70,27 +70,49 @@ func (ei *environmentIntegration) SetupOnce(client *Client) {
 }
 
 func (ei *environmentIntegration) processor(event *Event, hint *EventHint) *Event {
+	// Initialize maps as necessary.
 	if event.Contexts == nil {
 		event.Contexts = make(map[string]interface{})
 	}
-
-	event.Contexts["device"] = map[string]interface{}{
-		"arch":    runtime.GOARCH,
-		"num_cpu": runtime.NumCPU(),
+	for _, name := range []string{"device", "os", "runtime"} {
+		if event.Contexts[name] == nil {
+			event.Contexts[name] = make(map[string]interface{})
+		}
 	}
 
-	event.Contexts["os"] = map[string]interface{}{
-		"name": runtime.GOOS,
+	// Set contextual information preserving existing data. For each context, if
+	// the existing value is not of type map[string]interface{}, then no
+	// additional information is added.
+	if deviceContext, ok := event.Contexts["device"].(map[string]interface{}); ok {
+		if _, ok := deviceContext["arch"]; !ok {
+			deviceContext["arch"] = runtime.GOARCH
+		}
+		if _, ok := deviceContext["num_cpu"]; !ok {
+			deviceContext["num_cpu"] = runtime.NumCPU()
+		}
 	}
-
-	event.Contexts["runtime"] = map[string]interface{}{
-		"name":           "go",
-		"version":        runtime.Version(),
-		"go_numroutines": runtime.NumGoroutine(),
-		"go_maxprocs":    runtime.GOMAXPROCS(0),
-		"go_numcgocalls": runtime.NumCgoCall(),
+	if osContext, ok := event.Contexts["os"].(map[string]interface{}); ok {
+		if _, ok := osContext["name"]; !ok {
+			osContext["name"] = runtime.GOOS
+		}
+	}
+	if runtimeContext, ok := event.Contexts["runtime"].(map[string]interface{}); ok {
+		if _, ok := runtimeContext["name"]; !ok {
+			runtimeContext["name"] = "go"
+		}
+		if _, ok := runtimeContext["version"]; !ok {
+			runtimeContext["version"] = runtime.Version()
+		}
+		if _, ok := runtimeContext["go_numroutines"]; !ok {
+			runtimeContext["go_numroutines"] = runtime.NumGoroutine()
+		}
+		if _, ok := runtimeContext["go_maxprocs"]; !ok {
+			runtimeContext["go_maxprocs"] = runtime.GOMAXPROCS(0)
+		}
+		if _, ok := runtimeContext["go_numcgocalls"]; !ok {
+			runtimeContext["go_numcgocalls"] = runtime.NumCgoCall()
+		}
 	}
-
 	return event
 }
 
diff --git a/integrations_test.go b/integrations_test.go
index 4f130a7ef..83d2b4a3a 100644
--- a/integrations_test.go
+++ b/integrations_test.go
@@ -1,6 +1,7 @@
 package sentry
 
 import (
+	"encoding/json"
 	"path/filepath"
 	"regexp"
 	"runtime/debug"
@@ -342,3 +343,48 @@ func TestExtractModules(t *testing.T) {
 		})
 	}
 }
+
+func TestEnvironmentIntegrationDoesNotOverrideExistingContexts(t *testing.T) {
+	transport := &TransportMock{}
+	client, err := NewClient(ClientOptions{
+		Transport: transport,
+		Integrations: func([]Integration) []Integration {
+			return []Integration{new(environmentIntegration)}
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	scope := NewScope()
+
+	scope.contexts["device"] = map[string]interface{}{
+		"foo": "bar",
+	}
+	scope.contexts["os"] = map[string]interface{}{
+		"name": "test",
+	}
+	scope.contexts["custom"] = "value"
+	hub := NewHub(client, scope)
+	hub.CaptureMessage("test event")
+
+	events := transport.Events()
+	if len(events) != 1 {
+		b, err := json.MarshalIndent(events, "", "  ")
+		if err != nil {
+			t.Fatal(err)
+		}
+		t.Fatalf("events = %s\ngot %d events, want 1", b, len(events))
+	}
+
+	contexts := events[0].Contexts
+
+	if contexts["device"].(map[string]interface{})["foo"] != "bar" {
+		t.Errorf(`contexts["device"] = %#v, want contexts["device"]["foo"] == "bar"`, contexts["device"])
+	}
+	if contexts["os"].(map[string]interface{})["name"] != "test" {
+		t.Errorf(`contexts["os"] = %#v, want contexts["os"]["name"] == "test"`, contexts["os"])
+	}
+	if contexts["custom"] != "value" {
+		t.Errorf(`contexts["custom"] = %#v, want "value"`, contexts["custom"])
+	}
+}