diff --git a/api/src/main/java/org/apache/iceberg/PartitionSpec.java b/api/src/main/java/org/apache/iceberg/PartitionSpec.java
index 288445f82a17..e5f6e08bf9f1 100644
--- a/api/src/main/java/org/apache/iceberg/PartitionSpec.java
+++ b/api/src/main/java/org/apache/iceberg/PartitionSpec.java
@@ -26,6 +26,7 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicInteger;
 import org.apache.iceberg.exceptions.ValidationException;
@@ -362,7 +363,7 @@ Builder checkConflicts(boolean check) {
     }
 
     private void checkAndAddPartitionName(String name, Integer sourceColumnId) {
-      Types.NestedField schemaField = schema.findField(name);
+      Types.NestedField schemaField = schema.caseInsensitiveFindField(name);
       if (checkConflicts) {
         if (sourceColumnId != null) {
           // for identity transform case we allow conflicts between partition and schema field name as
@@ -397,114 +398,168 @@ public Builder withSpecId(int newSpecId) {
     }
 
     private Types.NestedField findSourceColumn(String sourceName) {
-      Types.NestedField sourceColumn = schema.findField(sourceName);
+      Types.NestedField sourceColumn = schema.caseInsensitiveFindField(sourceName);
       Preconditions.checkArgument(sourceColumn != null, "Cannot find source column: %s", sourceName);
       return sourceColumn;
     }
 
     Builder identity(String sourceName, String targetName) {
+      Preconditions.checkArgument(targetName == null, "targetName cannot be null");
+      return identity(sourceName, Optional.of(targetName));
+    }
+
+    private Builder identity(String sourceName, Optional<String> targetName) {
       Types.NestedField sourceColumn = findSourceColumn(sourceName);
-      checkAndAddPartitionName(targetName, sourceColumn.fieldId());
+      String targetPartitionName = targetName.orElseGet(sourceColumn::name);
+      checkAndAddPartitionName(targetPartitionName, sourceColumn.fieldId());
       PartitionField field = new PartitionField(
-          sourceColumn.fieldId(), nextFieldId(), targetName, Transforms.identity(sourceColumn.type()));
+          sourceColumn.fieldId(), nextFieldId(), targetPartitionName, Transforms.identity(sourceColumn.type()));
       checkForRedundantPartitions(field);
       fields.add(field);
       return this;
     }
 
     public Builder identity(String sourceName) {
-      return identity(sourceName, sourceName);
+      return identity(sourceName, Optional.empty());
     }
 
     public Builder year(String sourceName, String targetName) {
-      checkAndAddPartitionName(targetName);
+      Preconditions.checkArgument(targetName == null, "targetName cannot be null");
+      return year(sourceName, Optional.of(targetName));
+    }
+
+    private Builder year(String sourceName, Optional<String> targetName) {
       Types.NestedField sourceColumn = findSourceColumn(sourceName);
+      String targetPartitionName = targetName.orElseGet(() -> sourceColumn.name() + "_year");
+      checkAndAddPartitionName(targetPartitionName);
       PartitionField field = new PartitionField(
-          sourceColumn.fieldId(), nextFieldId(), targetName, Transforms.year(sourceColumn.type()));
+          sourceColumn.fieldId(), nextFieldId(), targetPartitionName, Transforms.year(sourceColumn.type()));
       checkForRedundantPartitions(field);
       fields.add(field);
       return this;
     }
 
     public Builder year(String sourceName) {
-      return year(sourceName, sourceName + "_year");
+      return year(sourceName, Optional.empty());
     }
 
     public Builder month(String sourceName, String targetName) {
-      checkAndAddPartitionName(targetName);
+      Preconditions.checkArgument(targetName == null, "targetName cannot be null");
+      return month(sourceName, Optional.of(targetName));
+    }
+    private Builder month(String sourceName, Optional<String> targetName) {
       Types.NestedField sourceColumn = findSourceColumn(sourceName);
+      String targetPartitionName = targetName.orElseGet(() -> sourceColumn.name() + "_month");
+      checkAndAddPartitionName(targetPartitionName);
       PartitionField field = new PartitionField(
-          sourceColumn.fieldId(), nextFieldId(), targetName, Transforms.month(sourceColumn.type()));
+          sourceColumn.fieldId(), nextFieldId(), targetPartitionName, Transforms.month(sourceColumn.type()));
       checkForRedundantPartitions(field);
       fields.add(field);
       return this;
     }
 
     public Builder month(String sourceName) {
-      return month(sourceName, sourceName + "_month");
+      return month(sourceName, Optional.empty());
     }
 
     public Builder day(String sourceName, String targetName) {
-      checkAndAddPartitionName(targetName);
+      Preconditions.checkArgument(targetName == null, "targetName cannot be null");
+      return day(sourceName, Optional.of(targetName));
+    }
+
+    private Builder day(String sourceName, Optional<String> targetName) {
       Types.NestedField sourceColumn = findSourceColumn(sourceName);
+      String targetPartitionName = targetName.orElseGet(() -> sourceColumn.name() + "_day");
+      checkAndAddPartitionName(targetPartitionName);
       PartitionField field = new PartitionField(
-          sourceColumn.fieldId(), nextFieldId(), targetName, Transforms.day(sourceColumn.type()));
+          sourceColumn.fieldId(), nextFieldId(), targetPartitionName, Transforms.day(sourceColumn.type()));
       checkForRedundantPartitions(field);
       fields.add(field);
       return this;
     }
 
     public Builder day(String sourceName) {
-      return day(sourceName, sourceName + "_day");
+      return day(sourceName, Optional.empty());
     }
 
     public Builder hour(String sourceName, String targetName) {
-      checkAndAddPartitionName(targetName);
+      Preconditions.checkArgument(targetName == null, "targetName cannot be null");
+      return hour(sourceName, Optional.of(targetName));
+    }
+
+    private Builder hour(String sourceName, Optional<String> targetName) {
       Types.NestedField sourceColumn = findSourceColumn(sourceName);
+      String targetPartitionName = targetName.orElseGet(() -> sourceColumn.name() + "_hour");
+      checkAndAddPartitionName(targetPartitionName);
       PartitionField field = new PartitionField(
-          sourceColumn.fieldId(), nextFieldId(), targetName, Transforms.hour(sourceColumn.type()));
+          sourceColumn.fieldId(), nextFieldId(), targetPartitionName, Transforms.hour(sourceColumn.type()));
       checkForRedundantPartitions(field);
       fields.add(field);
       return this;
     }
 
     public Builder hour(String sourceName) {
-      return hour(sourceName, sourceName + "_hour");
+      return hour(sourceName, Optional.empty());
     }
 
     public Builder bucket(String sourceName, int numBuckets, String targetName) {
-      checkAndAddPartitionName(targetName);
+      Preconditions.checkArgument(targetName == null, "targetName cannot be null");
+      return bucket(sourceName, numBuckets, Optional.of(targetName));
+    }
+
+    private Builder bucket(String sourceName, int numBuckets, Optional<String> targetName) {
       Types.NestedField sourceColumn = findSourceColumn(sourceName);
+      String targetPartitionName = targetName.orElseGet(() -> sourceColumn.name() + "_bucket");
+      checkAndAddPartitionName(targetPartitionName);
       fields.add(new PartitionField(
-          sourceColumn.fieldId(), nextFieldId(), targetName, Transforms.bucket(sourceColumn.type(), numBuckets)));
+              sourceColumn.fieldId(),
+              nextFieldId(),
+              targetPartitionName,
+              Transforms.bucket(sourceColumn.type(), numBuckets)));
       return this;
     }
 
     public Builder bucket(String sourceName, int numBuckets) {
-      return bucket(sourceName, numBuckets, sourceName + "_bucket");
+      return bucket(sourceName, numBuckets, Optional.empty());
     }
 
     public Builder truncate(String sourceName, int width, String targetName) {
-      checkAndAddPartitionName(targetName);
+      Preconditions.checkArgument(targetName == null, "targetName cannot be null");
+      return truncate(sourceName, width, Optional.of(targetName));
+    }
+
+    private Builder truncate(String sourceName, int width, Optional<String> targetName) {
       Types.NestedField sourceColumn = findSourceColumn(sourceName);
+      String targetPartitionName = targetName.orElseGet(() -> sourceColumn.name() + "_trunc");
+      checkAndAddPartitionName(targetPartitionName);
       fields.add(new PartitionField(
-          sourceColumn.fieldId(), nextFieldId(), targetName, Transforms.truncate(sourceColumn.type(), width)));
+          sourceColumn.fieldId(), nextFieldId(), targetPartitionName, Transforms.truncate(sourceColumn.type(), width)));
       return this;
     }
 
     public Builder truncate(String sourceName, int width) {
-      return truncate(sourceName, width, sourceName + "_trunc");
+      return truncate(sourceName, width, Optional.empty());
     }
 
     public Builder alwaysNull(String sourceName, String targetName) {
+      Preconditions.checkArgument(targetName == null, "targetName cannot be null");
+      return alwaysNull(sourceName, Optional.of(targetName));
+    }
+
+    private Builder alwaysNull(String sourceName, Optional<String> targetName) {
       Types.NestedField sourceColumn = findSourceColumn(sourceName);
-      checkAndAddPartitionName(targetName, sourceColumn.fieldId()); // can duplicate a source column name
-      fields.add(new PartitionField(sourceColumn.fieldId(), nextFieldId(), targetName, Transforms.alwaysNull()));
+      String targetPartitionName = targetName.orElseGet(() -> sourceColumn.name() + "_null");
+      checkAndAddPartitionName(targetPartitionName, sourceColumn.fieldId()); // can duplicate a source column name
+      fields.add(new PartitionField(
+              sourceColumn.fieldId(),
+              nextFieldId(),
+              targetPartitionName,
+              Transforms.alwaysNull()));
       return this;
     }
 
     public Builder alwaysNull(String sourceName) {
-      return alwaysNull(sourceName, sourceName + "_null");
+      return alwaysNull(sourceName, Optional.empty());
     }
 
     // add a partition field with an auto-increment partition field id starting from PARTITION_DATA_ID_START
diff --git a/api/src/test/java/org/apache/iceberg/TestPartitionSpecValidation.java b/api/src/test/java/org/apache/iceberg/TestPartitionSpecValidation.java
index 4834e63903bd..febfe71dd496 100644
--- a/api/src/test/java/org/apache/iceberg/TestPartitionSpecValidation.java
+++ b/api/src/test/java/org/apache/iceberg/TestPartitionSpecValidation.java
@@ -29,9 +29,11 @@ public class TestPartitionSpecValidation {
       NestedField.required(1, "id", Types.LongType.get()),
       NestedField.required(2, "ts", Types.TimestampType.withZone()),
       NestedField.required(3, "another_ts", Types.TimestampType.withZone()),
-      NestedField.required(4, "d", Types.TimestampType.withZone()),
-      NestedField.required(5, "another_d", Types.TimestampType.withZone()),
-      NestedField.required(6, "s", Types.StringType.get())
+      NestedField.required(4, "mIxEd_CAse_ts", Types.TimestampType.withZone()),
+      NestedField.required(5, "d", Types.TimestampType.withZone()),
+      NestedField.required(6, "another_d", Types.TimestampType.withZone()),
+      NestedField.required(7, "s", Types.StringType.get()),
+      NestedField.required(8, "mIxEd_CAse_s", Types.StringType.get())
   );
 
   @Test
@@ -263,4 +265,31 @@ public void testAddPartitionFieldsWithAndWithoutFieldIds() {
     Assert.assertEquals(1006, spec.fields().get(2).fieldId());
     Assert.assertEquals(1006, spec.lastAssignedFieldId());
   }
+
+  @Test
+  public void testSettingPartitionTransformsWithCaseInsensitiveSourceColumnNames() {
+    Assert.assertEquals(PartitionSpec.builderFor(SCHEMA).year("mixed_case_ts")
+            .build().fields().get(0).name(), "mIxEd_CAse_ts_year");
+    Assert.assertEquals(PartitionSpec.builderFor(SCHEMA).month("mixed_case_ts")
+            .build().fields().get(0).name(), "mIxEd_CAse_ts_month");
+    Assert.assertEquals(PartitionSpec.builderFor(SCHEMA).day("mixed_case_ts")
+            .build().fields().get(0).name(), "mIxEd_CAse_ts_day");
+    Assert.assertEquals(PartitionSpec.builderFor(SCHEMA).hour("mixed_case_ts")
+            .build().fields().get(0).name(), "mIxEd_CAse_ts_hour");
+    Assert.assertEquals(PartitionSpec.builderFor(SCHEMA)
+            .bucket("mixed_case_ts", 4)
+            .build().fields().get(0).name(), "mIxEd_CAse_ts_bucket");
+    Assert.assertEquals(PartitionSpec.builderFor(SCHEMA)
+            .truncate("S", 1)
+            .build().fields().get(0).name(), "s_trunc");
+    Assert.assertEquals(PartitionSpec.builderFor(SCHEMA)
+            .truncate("mixed_case_s", 1)
+            .build().fields().get(0).name(), "mIxEd_CAse_s_trunc");
+    Assert.assertEquals(PartitionSpec.builderFor(SCHEMA)
+            .identity("mixed_case_s")
+            .build().fields().get(0).name(), "mIxEd_CAse_s");
+    Assert.assertEquals(PartitionSpec.builderFor(SCHEMA)
+            .alwaysNull("mixed_case_s")
+            .build().fields().get(0).name(), "mIxEd_CAse_s_null");
+  }
 }