diff --git a/lang/java/avro/src/main/java/org/apache/avro/io/FastReaderBuilder.java b/lang/java/avro/src/main/java/org/apache/avro/io/FastReaderBuilder.java
index f8d66c7069b..dbadd534ac1 100644
--- a/lang/java/avro/src/main/java/org/apache/avro/io/FastReaderBuilder.java
+++ b/lang/java/avro/src/main/java/org/apache/avro/io/FastReaderBuilder.java
@@ -124,6 +124,14 @@ private FieldReader getReaderFor(Schema readerSchema, Schema writerSchema) throw
   }
 
   private FieldReader getReaderFor(Action action, Conversion<?> explicitConversion) throws IOException {
+    // A writer union must not be wrapped in a conversion here: the value is
+    // produced by
+    // the branch reader selected at read time, so the conversion has to be applied
+    // by that
+    // branch reader, using the branch's own reader schema, exactly once.
+    if (action instanceof WriterUnion) {
+      return createUnionReader((WriterUnion) action, explicitConversion);
+    }
     final FieldReader baseReader = getNonConvertedReader(action);
     return applyConversions(action.reader, baseReader, explicitConversion);
   }
@@ -294,8 +302,6 @@ private FieldReader getNonConvertedReader(Action action) throws IOException {
       return createEnumReader((EnumAdjust) action);
     case PROMOTE:
       return createPromotingReader((Promote) action);
-    case WRITER_UNION:
-      return createUnionReader((WriterUnion) action);
     case READER_UNION:
       return getReaderFor(((ReaderUnion) action).actualAction, null);
     case ERROR:
@@ -409,10 +415,13 @@ private Utf8 getUtf8FromByteBuffer(Object old, ByteBuffer buffer) {
     return (old instanceof Utf8) ? ((Utf8) old).set(new Utf8(buffer.array())) : new Utf8(buffer.array());
   }
 
-  private FieldReader createUnionReader(WriterUnion action) throws IOException {
+  private FieldReader createUnionReader(WriterUnion action, Conversion<?> explicitConversion) throws IOException {
     FieldReader[] unionReaders = new FieldReader[action.actions.length];
     for (int i = 0; i < action.actions.length; i++) {
-      unionReaders[i] = getReaderFor(action.actions[i], null);
+      // the conversion is owned by the branch: the branch's reader schema decides
+      // whether
+      // (and which of) the logical type conversions apply
+      unionReaders[i] = getReaderFor(action.actions[i], explicitConversion);
     }
     return createUnionReader(unionReaders);
   }
diff --git a/lang/java/avro/src/test/java/org/apache/avro/io/TestFastReaderBuilder.java b/lang/java/avro/src/test/java/org/apache/avro/io/TestFastReaderBuilder.java
new file mode 100644
index 00000000000..f87536b3143
--- /dev/null
+++ b/lang/java/avro/src/test/java/org/apache/avro/io/TestFastReaderBuilder.java
@@ -0,0 +1,270 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.avro.io;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertInstanceOf;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.time.LocalDate;
+
+import org.apache.avro.Conversion;
+import org.apache.avro.LogicalType;
+import org.apache.avro.LogicalTypes;
+import org.apache.avro.Schema;
+import org.apache.avro.SchemaBuilder;
+import org.apache.avro.data.TimeConversions.DateConversion;
+import org.apache.avro.generic.GenericData;
+import org.apache.avro.generic.GenericDatumReader;
+import org.apache.avro.generic.GenericDatumWriter;
+import org.apache.avro.specific.SpecificData;
+import org.apache.avro.specific.SpecificRecordBase;
+import org.junit.jupiter.api.Test;
+
+/**
+ * AVRO-4215: when the writer schema selects the value through a union, the
+ * logical type conversion must be applied exactly once.
+ */
+public class TestFastReaderBuilder {
+
+  private static final int EPOCH_DAYS = 1000;
+
+  private static final LocalDate EXPECTED_DATE = LocalDate.ofEpochDay(EPOCH_DAYS);
+
+  private static final Schema WRITER_INT = Schema.create(Schema.Type.INT);
+
+  private static final Schema WRITER_UNION_INT = SchemaBuilder.unionOf().intType().endUnion();
+
+  private static final Schema WRITER_NULLABLE_INT = SchemaBuilder.unionOf().nullType().and().intType().endUnion();
+
+  private static final Schema READER_DATE = LogicalTypes.date().addToSchema(Schema.create(Schema.Type.INT));
+
+  private static final Schema READER_NULLABLE_DATE = SchemaBuilder.unionOf().nullType().and().type(READER_DATE)
+      .endUnion();
+
+  private static final Schema WRITER_UNION_INT_RECORD = SchemaBuilder.record("DateRecord")
+      .namespace("org.apache.avro.io.TestFastReaderBuilder").fields().name("date").type(WRITER_UNION_INT).noDefault()
+      .endRecord();
+
+  private static final Schema READER_DATE_RECORD = SchemaBuilder.record("DateRecord")
+      .namespace("org.apache.avro.io.TestFastReaderBuilder").fields().name("date").type(READER_DATE).noDefault()
+      .endRecord();
+
+  private static GenericData newDataModel(boolean fastReaderEnabled) {
+    GenericData model = new GenericData();
+    model.addLogicalTypeConversion(new DateConversion());
+    model.setFastReaderEnabled(fastReaderEnabled);
+    return model;
+  }
+
+  private static byte[] write(Schema writerSchema, GenericData model, Object datum) throws IOException {
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+    Encoder encoder = EncoderFactory.get().binaryEncoder(out, null);
+    new GenericDatumWriter<>(writerSchema, model).write(datum, encoder);
+    encoder.flush();
+    return out.toByteArray();
+  }
+
+  private static Object read(byte[] serialized, Schema writerSchema, Schema readerSchema, GenericData model)
+      throws IOException {
+    try (ByteArrayInputStream in = new ByteArrayInputStream(serialized)) {
+      Decoder decoder = DecoderFactory.get().directBinaryDecoder(in, null);
+      GenericDatumReader<Object> reader = new GenericDatumReader<>(writerSchema, readerSchema, model);
+      return reader.read(null, decoder);
+    }
+  }
+
+  @Test
+  void writerUnionIntToDateWithFastReader() throws IOException {
+    GenericData model = newDataModel(true);
+    byte[] serialized = write(WRITER_UNION_INT, model, EPOCH_DAYS);
+
+    Object datum = read(serialized, WRITER_UNION_INT, READER_DATE, model);
+
+    assertInstanceOf(LocalDate.class, datum);
+    assertEquals(EXPECTED_DATE, datum);
+  }
+
+  @Test
+  void writerUnionIntToDateWithResolvingDecoder() throws IOException {
+    GenericData model = newDataModel(false);
+    byte[] serialized = write(WRITER_UNION_INT, model, EPOCH_DAYS);
+
+    Object datum = read(serialized, WRITER_UNION_INT, READER_DATE, model);
+
+    assertInstanceOf(LocalDate.class, datum);
+    assertEquals(EXPECTED_DATE, datum);
+  }
+
+  @Test
+  void writerUnionIntToDateWithoutRegisteredConversion() throws IOException {
+    GenericData model = new GenericData();
+    model.setFastReaderEnabled(true);
+    byte[] serialized = write(WRITER_UNION_INT, model, EPOCH_DAYS);
+
+    assertEquals(EPOCH_DAYS, read(serialized, WRITER_UNION_INT, READER_DATE, model));
+  }
+
+  @Test
+  void readerUnionDateWithFastReader() throws IOException {
+    GenericData model = newDataModel(true);
+    byte[] serialized = write(WRITER_INT, model, EPOCH_DAYS);
+
+    Object datum = read(serialized, WRITER_INT, READER_NULLABLE_DATE, model);
+
+    assertInstanceOf(LocalDate.class, datum);
+    assertEquals(EXPECTED_DATE, datum);
+  }
+
+  @Test
+  void nullableWriterUnionIntToDateWithFastReader() throws IOException {
+    GenericData model = newDataModel(true);
+
+    Object date = read(write(WRITER_NULLABLE_INT, model, EPOCH_DAYS), WRITER_NULLABLE_INT, READER_NULLABLE_DATE, model);
+    assertInstanceOf(LocalDate.class, date);
+    assertEquals(EXPECTED_DATE, date);
+
+    Object nullDatum = read(write(WRITER_NULLABLE_INT, model, null), WRITER_NULLABLE_INT, READER_NULLABLE_DATE, model);
+    assertNull(nullDatum);
+  }
+
+  @Test
+  void nullableWriterUnionIntToDateWithResolvingDecoder() throws IOException {
+    GenericData model = newDataModel(false);
+
+    Object date = read(write(WRITER_NULLABLE_INT, model, EPOCH_DAYS), WRITER_NULLABLE_INT, READER_NULLABLE_DATE, model);
+    assertInstanceOf(LocalDate.class, date);
+    assertEquals(EXPECTED_DATE, date);
+
+    Object nullDatum = read(write(WRITER_NULLABLE_INT, model, null), WRITER_NULLABLE_INT, READER_NULLABLE_DATE, model);
+    assertNull(nullDatum);
+  }
+
+  @Test
+  void specificRecordUsesFieldConversionOnceForWriterUnion() throws IOException {
+    CountingDateConversion registeredConversion = new CountingDateConversion();
+    CountingDateConversion fieldConversion = new CountingDateConversion();
+    IsolatedSpecificData model = new IsolatedSpecificData(registeredConversion, fieldConversion);
+    model.setFastReaderEnabled(true);
+
+    GenericData.Record writerDatum = new GenericData.Record(WRITER_UNION_INT_RECORD);
+    writerDatum.put("date", EPOCH_DAYS);
+    byte[] serialized = write(WRITER_UNION_INT_RECORD, new GenericData(), writerDatum);
+
+    Object datum;
+    try (ByteArrayInputStream in = new ByteArrayInputStream(serialized)) {
+      Decoder decoder = DecoderFactory.get().directBinaryDecoder(in, null);
+      DatumReader<Object> reader = new FastReaderBuilder(model).createDatumReader(WRITER_UNION_INT_RECORD,
+          READER_DATE_RECORD);
+      datum = reader.read(null, decoder);
+    }
+
+    assertInstanceOf(DateRecord.class, datum);
+    assertEquals(EXPECTED_DATE, ((DateRecord) datum).get(0));
+    assertEquals(1, fieldConversion.calls());
+    assertEquals(0, registeredConversion.calls());
+  }
+
+  private static class CountingDateConversion extends DateConversion {
+    private int calls;
+
+    @Override
+    public LocalDate fromInt(Integer value, Schema schema, LogicalType type) {
+      calls++;
+      return super.fromInt(value, schema, type);
+    }
+
+    int calls() {
+      return calls;
+    }
+  }
+
+  /**
+   * Hand-written specific record; the schema namespace is the enclosing class so
+   * the class can be resolved as a nested class.
+   */
+  public static class DateRecord extends SpecificRecordBase {
+    public static final Schema SCHEMA$ = READER_DATE_RECORD;
+
+    private final Conversion<?> fieldConversion;
+
+    private LocalDate date;
+
+    public DateRecord() {
+      this(null);
+    }
+
+    DateRecord(Conversion<?> fieldConversion) {
+      this.fieldConversion = fieldConversion;
+    }
+
+    public static Schema getClassSchema() {
+      return SCHEMA$;
+    }
+
+    @Override
+    public Schema getSchema() {
+      return SCHEMA$;
+    }
+
+    @Override
+    public Object get(int field) {
+      if (field != 0) {
+        throw new IndexOutOfBoundsException("Invalid field index: " + field);
+      }
+      return date;
+    }
+
+    @Override
+    public void put(int field, Object value) {
+      if (field != 0) {
+        throw new IndexOutOfBoundsException("Invalid field index: " + field);
+      }
+      this.date = (LocalDate) value;
+    }
+
+    @Override
+    public Conversion<?> getConversion(int field) {
+      return field == 0 ? fieldConversion : null;
+    }
+  }
+
+  /** Isolated data model: never mutates the global SpecificData singleton. */
+  private static final class IsolatedSpecificData extends SpecificData {
+    private final Conversion<?> fieldConversion;
+
+    IsolatedSpecificData(CountingDateConversion registeredConversion, Conversion<?> fieldConversion) {
+      this.fieldConversion = fieldConversion;
+      if (registeredConversion != null) {
+        addLogicalTypeConversion(registeredConversion);
+      }
+    }
+
+    @Override
+    public GenericData.InstanceSupplier getNewRecordSupplier(Schema schema) {
+      if (DateRecord.SCHEMA$.getFullName().equals(schema.getFullName())) {
+        final Conversion<?> conversion = fieldConversion;
+        return (old, targetSchema) -> new DateRecord(conversion);
+      }
+      return super.getNewRecordSupplier(schema);
+    }
+  }
+}