diff --git a/google-cloud-clients/google-cloud-spanner/pom.xml b/google-cloud-clients/google-cloud-spanner/pom.xml
index 3a563fe7eddd..ad5981934bf0 100644
--- a/google-cloud-clients/google-cloud-spanner/pom.xml
+++ b/google-cloud-clients/google-cloud-spanner/pom.xml
@@ -18,6 +18,7 @@
   </parent>
   <properties>
     <site.installationModule>google-cloud-spanner</site.installationModule>
+    <skipUnitTests>${skipTests}</skipUnitTests>
   </properties>
   <build>
     <plugins>
@@ -27,6 +28,7 @@
         <version>2.12.4</version>
         <configuration>
           <excludedGroups>com.google.cloud.spanner.IntegrationTest</excludedGroups>
+          <skipTests>${skipUnitTests}</skipTests>
         </configuration>
       </plugin>
       <plugin>
diff --git a/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java b/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java
index 850bf2898f57..f17c97dbcf45 100644
--- a/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java
+++ b/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java
@@ -81,7 +81,6 @@
 import io.opencensus.trace.Span;
 import io.opencensus.trace.Tracer;
 import io.opencensus.trace.Tracing;
-
 import java.io.IOException;
 import java.io.Serializable;
 import java.util.AbstractList;
@@ -129,6 +128,19 @@ class SpannerImpl extends BaseService<SpannerOptions> implements Spanner {
   private static final String QUERY = "CloudSpannerOperation.ExecuteStreamingQuery";
   private static final String READ = "CloudSpannerOperation.ExecuteStreamingRead";
 
+  private static final ThreadLocal<Boolean> hasPendingTransaction = new ThreadLocal<Boolean>() {
+    @Override
+    protected Boolean initialValue() {
+      return false;
+    }
+  };
+
+  private static void throwIfTransactionsPending() {
+    if (hasPendingTransaction.get() == Boolean.TRUE) {
+        throw newSpannerException(ErrorCode.INTERNAL, "Nested transactions are not supported");
+    }
+  }
+
   static {
     TraceUtil.exportSpans(CREATE_SESSION, DELETE_SESSION, BEGIN_TRANSACTION, COMMIT, QUERY, READ);
   }
@@ -905,6 +917,8 @@ TransactionContextImpl newTransaction() {
     }
     
     <T extends SessionTransaction> T setActive(@Nullable T ctx) {
+      throwIfTransactionsPending();
+
       if (activeTransaction != null) {
         activeTransaction.invalidate();
       }
@@ -1239,11 +1253,13 @@ void backoffSleep(Context context, long backoffMillis) {
     @Override
     public <T> T run(TransactionCallable<T> callable) {
       try (Scope s = tracer.withSpan(span)) {
+        hasPendingTransaction.set(Boolean.TRUE);
         return runInternal(callable);
       } catch (RuntimeException e) {
         TraceUtil.endSpanWithFailure(span, e);
         throw e;
       } finally {
+        hasPendingTransaction.set(Boolean.FALSE);
         span.end();
       }
     }
@@ -1660,6 +1676,8 @@ ByteString getTransactionId() {
     }
 
     void initTransaction() {
+      throwIfTransactionsPending();
+
       // Since we only support synchronous calls, just block on "txnLock" while the RPC is in
       // flight.  Note that we use the strategy of sending an explicit BeginTransaction() RPC,
       // rather than using the first read in the transaction to begin it implicitly.  The chosen
diff --git a/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/TransactionRunnerImplTest.java b/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/TransactionRunnerImplTest.java
index 9362814146ab..0cd804c6a89e 100644
--- a/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/TransactionRunnerImplTest.java
+++ b/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/TransactionRunnerImplTest.java
@@ -32,7 +32,6 @@
 import io.grpc.Status;
 import io.grpc.StatusRuntimeException;
 import java.util.concurrent.atomic.AtomicInteger;
-
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
diff --git a/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITTransactionTest.java b/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITTransactionTest.java
index 1bacea50d627..139c75f66a04 100644
--- a/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITTransactionTest.java
+++ b/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITTransactionTest.java
@@ -23,13 +23,17 @@
 
 import com.google.cloud.Timestamp;
 import com.google.cloud.spanner.AbortedException;
+import com.google.cloud.spanner.BatchClient;
+import com.google.cloud.spanner.BatchReadOnlyTransaction;
 import com.google.cloud.spanner.Database;
 import com.google.cloud.spanner.DatabaseClient;
 import com.google.cloud.spanner.ErrorCode;
 import com.google.cloud.spanner.IntegrationTest;
 import com.google.cloud.spanner.IntegrationTestEnv;
 import com.google.cloud.spanner.Key;
+import com.google.cloud.spanner.KeySet;
 import com.google.cloud.spanner.Mutation;
+import com.google.cloud.spanner.PartitionOptions;
 import com.google.cloud.spanner.ReadContext;
 import com.google.cloud.spanner.ResultSet;
 import com.google.cloud.spanner.SpannerException;
@@ -349,4 +353,110 @@ public Void run(TransactionContext transaction) throws SpannerException {
                 .getLong(0))
         .isEqualTo(2);
   }
+
+  private void doNestedRwTransaction() {
+    client
+        .readWriteTransaction()
+        .run(
+            new TransactionCallable<Void>() {
+              @Override
+              public Void run(TransactionContext transaction) throws SpannerException {
+                client
+                    .readWriteTransaction()
+                    .run(
+                        new TransactionCallable<Void>() {
+                          @Override
+                          public Void run(TransactionContext transaction) throws Exception {
+                            return null;
+                          }
+                        });
+
+                return null;
+              }
+            });
+  }
+
+  @Test
+  public void nestedRwRwTransactionShouldThrowException() {
+    try {
+      doNestedRwTransaction();
+      fail("Expected exception");
+    } catch (SpannerException e) {
+      assertThat(e.getErrorCode()).isEqualTo(ErrorCode.INTERNAL);
+      assertThat(e.getMessage()).contains("not supported");
+    }
+  }
+
+  @Test
+  public void nestedRwRdTransactionShouldThrowException() {
+    try {
+      client
+          .readWriteTransaction()
+          .run(
+              new TransactionCallable<Void>() {
+                @Override
+                public Void run(TransactionContext transaction) throws SpannerException {
+                  client
+                      .readOnlyTransaction()
+                      .getReadTimestamp();
+
+                  return null;
+                }
+              });
+      fail("Expected exception");
+    } catch (SpannerException e) {
+      assertThat(e.getErrorCode()).isEqualTo(ErrorCode.INTERNAL);
+      assertThat(e.getMessage()).contains("not supported");
+    }
+  }
+
+  @Test
+  public void nestedRwBatchTransactionShouldThrowException() {
+    try {
+      client
+          .readWriteTransaction()
+          .run(
+              new TransactionCallable<Void>() {
+                @Override
+                public Void run(TransactionContext transaction) throws SpannerException {
+                  BatchClient batchClient = env.getTestHelper().getBatchClient(db);
+                  BatchReadOnlyTransaction batchTxn = batchClient
+                      .batchReadOnlyTransaction(TimestampBound.strong());
+                  batchTxn.partitionReadUsingIndex(
+                      PartitionOptions.getDefaultInstance(),
+                      "Test",
+                      "Index",
+                      KeySet.all(),
+                      Arrays.asList("Fingerprint"));
+
+                  return null;
+                }
+              });
+      fail("Expected exception");
+    } catch (SpannerException e) {
+      assertThat(e.getErrorCode()).isEqualTo(ErrorCode.INTERNAL);
+      assertThat(e.getMessage()).contains("not supported");
+    }
+  }
+
+  @Test
+  public void nestedRwSingleUseReadTransactionShouldThrowException() {
+    try {
+      client
+          .readWriteTransaction()
+          .run(
+              new TransactionCallable<Void>() {
+                @Override
+                public Void run(TransactionContext transaction) throws SpannerException {
+                  client.singleUseReadOnlyTransaction();
+
+                  return null;
+                }
+              });
+      fail("Expected exception");
+    } catch (SpannerException e) {
+      assertThat(e.getErrorCode()).isEqualTo(ErrorCode.INTERNAL);
+      assertThat(e.getMessage()).contains("not supported");
+    }
+  }
 }