diff --git a/google-cloud-clients/google-cloud-spanner/pom.xml b/google-cloud-clients/google-cloud-spanner/pom.xml
index d6f7e78b68d4..60c795bdedab 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>${skipTests}</skipUnitTests>
     <jacoco.skip>true</jacoco.skip>
   </properties>
   <build>
@@ -47,6 +48,7 @@
         <version>2.12.4</version>
         <configuration>
           <excludedGroups>com.google.cloud.spanner.IntegrationTest</excludedGroups>
+          <skipTests>${skipUnitTests}</skipTests>
           <reportNameSuffix>sponge_log</reportNameSuffix>
         </configuration>
       </plugin>
diff --git a/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java b/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java
index 80e4d67eb664..ced479151253 100644
--- a/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java
+++ b/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java
@@ -408,6 +408,12 @@ public <T> T run(TransactionCallable<T> callable) {
         public Timestamp getCommitTimestamp() {
           return runner.getCommitTimestamp();
         }
+
+        @Override
+        public TransactionRunner allowNestedTransaction() {
+          runner.allowNestedTransaction();
+          return runner;
+        }
       };
     }
 
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..579da6797ced 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();
       }
@@ -1209,6 +1223,7 @@ public void execute(Runnable command) {
 
   @VisibleForTesting
   static class TransactionRunnerImpl implements SessionTransaction, TransactionRunner {
+    private boolean blockNestedTxn = true;
 
     /** Allow for testing of backoff logic */
     static class Sleeper {
@@ -1223,6 +1238,11 @@ void backoffSleep(Context context, long backoffMillis) {
     private TransactionContextImpl txn;
     private volatile boolean isValid = true;
 
+    public TransactionRunner allowNestedTransaction() {
+      blockNestedTxn = false;
+      return this;
+    }
+
     TransactionRunnerImpl(
         SessionImpl session, SpannerRpc rpc, Sleeper sleeper, int defaultPrefetchChunks) {
       this.session = session;
@@ -1239,11 +1259,19 @@ void backoffSleep(Context context, long backoffMillis) {
     @Override
     public <T> T run(TransactionCallable<T> callable) {
       try (Scope s = tracer.withSpan(span)) {
+        if (blockNestedTxn) {
+          hasPendingTransaction.set(Boolean.TRUE);
+        }
+
         return runInternal(callable);
       } catch (RuntimeException e) {
         TraceUtil.endSpanWithFailure(span, e);
         throw e;
       } finally {
+        // Remove threadLocal rather than set to FALSE to avoid a possible memory leak.
+        // We also do this unconditionally in case a user has modified the flag when the transaction
+        // was running.
+        hasPendingTransaction.remove();
         span.end();
       }
     }
@@ -1660,6 +1688,8 @@ 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/main/java/com/google/cloud/spanner/TransactionRunner.java b/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/TransactionRunner.java
index 65659ec98b7e..02d5947109d7 100644
--- a/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/TransactionRunner.java
+++ b/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/TransactionRunner.java
@@ -73,4 +73,23 @@ interface TransactionCallable<T> {
    * {@link #run(TransactionCallable)} has returned normally.
    */
   Timestamp getCommitTimestamp();
+
+  /**
+   * Allows overriding the default behaviour of blocking nested transactions.
+   *
+   * Note that the client library does not maintain any information regarding the nesting structure.
+   * If an outer transaction fails and an inner transaction succeeds, upon retry of the outer
+   * transaction, the inner transaction will be re-executed.
+   *
+   * Use with care when certain that the inner transaction is idempotent. Avoid doing this when
+   * accessing the same db. There might be legitimate uses where access need to be made across DBs
+   * for instance.
+   *
+   * E.g. of nesting that is discouraged, see {@code nestedReadWriteTxnThrows}
+   * {@code nestedReadOnlyTxnThrows}, {@code nestedBatchTxnThrows},
+   * {@code nestedSingleUseReadTxnThrows}
+   *
+   * @return this object
+   */
+  TransactionRunner allowNestedTransaction();
 }
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..c9fb40416fef 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 +353,126 @@ 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 nestedReadWriteTxnThrows() {
+    try {
+      doNestedRwTransaction();
+      fail("Expected exception");
+    } catch (SpannerException e) {
+      assertThat(e.getErrorCode()).isEqualTo(ErrorCode.INTERNAL);
+      assertThat(e.getMessage()).contains("not supported");
+    }
+  }
+
+  @Test
+  public void nestedReadOnlyTxnThrows() {
+    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 nestedBatchTxnThrows() {
+    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 nestedSingleUseReadTxnThrows() {
+    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");
+    }
+  }
+
+  @Test
+  public void nestedTxnSucceedsWhenAllowed() {
+    client
+        .readWriteTransaction()
+        .allowNestedTransaction()
+        .run(
+            new TransactionCallable<Void>() {
+              @Override
+              public Void run(TransactionContext transaction) throws SpannerException {
+                client.singleUseReadOnlyTransaction();
+
+                return null;
+              }
+            });
+  }
 }