diff --git a/google/cloud/spanner/options.h b/google/cloud/spanner/options.h
index 8111acfc5df15..663b605ae17ce 100644
--- a/google/cloud/spanner/options.h
+++ b/google/cloud/spanner/options.h
@@ -46,6 +46,7 @@
 #include "google/cloud/spanner/polling_policy.h"
 #include "google/cloud/spanner/request_priority.h"
 #include "google/cloud/spanner/retry_policy.h"
+#include "google/cloud/spanner/transaction.h"
 #include "google/cloud/spanner/version.h"
 #include "google/cloud/options.h"
 #include "absl/types/variant.h"
@@ -415,6 +416,15 @@ struct ExcludeTransactionFromChangeStreamsOption {
   using Type = bool;
 };
 
+/**
+ * Option for `google::cloud::Options` to set the transaction isolation level.
+ *
+ * @ingroup google-cloud-spanner-options
+ */
+struct TransactionIsolationLevelOption {
+  using Type = spanner::Transaction::IsolationLevel;
+};
+
 /**
  * Option for `google::cloud::Options` to return additional statistics
  * about the committed transaction in a `spanner::CommitResult`.
diff --git a/google/cloud/spanner/transaction.cc b/google/cloud/spanner/transaction.cc
index 07d9c9edd464d..457b21c943c76 100644
--- a/google/cloud/spanner/transaction.cc
+++ b/google/cloud/spanner/transaction.cc
@@ -54,6 +54,22 @@ ProtoReadLockMode(
   }
 }
 
+google::spanner::v1::TransactionOptions_IsolationLevel ProtoIsolationLevel(
+    absl::optional<Transaction::IsolationLevel> const& isolation_level) {
+  if (!isolation_level) {
+    return google::spanner::v1::TransactionOptions::ISOLATION_LEVEL_UNSPECIFIED;
+  }
+  switch (*isolation_level) {
+    case Transaction::IsolationLevel::kSerializable:
+      return google::spanner::v1::TransactionOptions::SERIALIZABLE;
+    case Transaction::IsolationLevel::kRepeatableRead:
+      return google::spanner::v1::TransactionOptions::REPEATABLE_READ;
+    default:
+      return google::spanner::v1::TransactionOptions::
+          ISOLATION_LEVEL_UNSPECIFIED;
+  }
+}
+
 google::spanner::v1::TransactionOptions MakeOpts(
     google::spanner::v1::TransactionOptions_ReadOnly ro_opts) {
   google::spanner::v1::TransactionOptions opts;
@@ -62,13 +78,22 @@ google::spanner::v1::TransactionOptions MakeOpts(
 }
 
 google::spanner::v1::TransactionOptions MakeOpts(
-    google::spanner::v1::TransactionOptions_ReadWrite rw_opts) {
+    google::spanner::v1::TransactionOptions_ReadWrite rw_opts,
+    absl::optional<Transaction::IsolationLevel> isolation_level) {
   google::spanner::v1::TransactionOptions opts;
   *opts.mutable_read_write() = std::move(rw_opts);
   auto const& current = internal::CurrentOptions();
   if (current.get<ExcludeTransactionFromChangeStreamsOption>()) {
     opts.set_exclude_txn_from_change_streams(true);
   }
+  if (isolation_level &&
+      *isolation_level != Transaction::IsolationLevel::kUnspecified) {
+    opts.set_isolation_level(ProtoIsolationLevel(isolation_level));
+  } else if (current.has<TransactionIsolationLevelOption>()) {
+    opts.set_isolation_level(
+        ProtoIsolationLevel(current.get<TransactionIsolationLevelOption>()));
+  }
+
   return opts;
 }
 
@@ -103,6 +128,13 @@ Transaction::ReadWriteOptions& Transaction::ReadWriteOptions::WithTag(
   return *this;
 }
 
+Transaction::ReadWriteOptions&
+Transaction::ReadWriteOptions::WithIsolationLevel(
+    IsolationLevel isolation_level) {
+  isolation_level_ = isolation_level;
+  return *this;
+}
+
 Transaction::SingleUseOptions::SingleUseOptions(ReadOnlyOptions opts) {
   ro_opts_ = std::move(opts.ro_opts_);
 }
@@ -129,7 +161,8 @@ Transaction::Transaction(ReadOnlyOptions opts) {
 
 Transaction::Transaction(ReadWriteOptions opts) {
   google::spanner::v1::TransactionSelector selector;
-  *selector.mutable_begin() = MakeOpts(std::move(opts.rw_opts_));
+  *selector.mutable_begin() =
+      MakeOpts(std::move(opts.rw_opts_), opts.isolation_level_);
   auto const route_to_leader = true;  // read-write
   impl_ = std::make_shared<spanner_internal::TransactionImpl>(
       std::move(selector), route_to_leader,
@@ -138,7 +171,8 @@ Transaction::Transaction(ReadWriteOptions opts) {
 
 Transaction::Transaction(Transaction const& txn, ReadWriteOptions opts) {
   google::spanner::v1::TransactionSelector selector;
-  *selector.mutable_begin() = MakeOpts(std::move(opts.rw_opts_));
+  *selector.mutable_begin() =
+      MakeOpts(std::move(opts.rw_opts_), opts.isolation_level_);
   auto const route_to_leader = true;  // read-write
   impl_ = std::make_shared<spanner_internal::TransactionImpl>(
       *txn.impl_, std::move(selector), route_to_leader,
@@ -155,7 +189,8 @@ Transaction::Transaction(SingleUseOptions opts) {
 
 Transaction::Transaction(ReadWriteOptions opts, SingleUseCommitTag) {
   google::spanner::v1::TransactionSelector selector;
-  *selector.mutable_single_use() = MakeOpts(std::move(opts.rw_opts_));
+  *selector.mutable_single_use() =
+      MakeOpts(std::move(opts.rw_opts_), opts.isolation_level_);
   auto const route_to_leader = true;  // write
   impl_ = std::make_shared<spanner_internal::TransactionImpl>(
       std::move(selector), route_to_leader,
diff --git a/google/cloud/spanner/transaction.h b/google/cloud/spanner/transaction.h
index 0949d52229a7f..2af203cba6fb0 100644
--- a/google/cloud/spanner/transaction.h
+++ b/google/cloud/spanner/transaction.h
@@ -57,6 +57,36 @@ GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
  */
 class Transaction {
  public:
+  /**
+   * Defines the isolation level for a transaction.
+   *
+   * This determines how concurrent transactions interact with each other and
+   * what consistency guarantees are provided for read and write operations.
+   * **Note:** This setting only applies to read-write transactions.
+   *
+   * See the `v1::TransactionOptions` proto for more details.
+   *
+   * @see https://docs.cloud.google.com/spanner/docs/isolation-levels
+   */
+  enum class IsolationLevel {
+    /// The isolation level is not specified, using the backend default.
+    kUnspecified,
+    /**
+     * All transactions appear as if they executed in a serial order.
+     * This is the default isolation level for read-write transactions.
+     */
+    kSerializable,
+    /**
+     * All reads performed during the transaction observe a consistent snapshot
+     * of the database. The transaction is only successfully committed in the
+     * absence of conflicts between its updates and any concurrent updates
+     * that have occurred since that snapshot. Consequently, in contrast to
+     * `kSerializable` transactions, only write-write conflicts are detected in
+     * snapshot transactions.
+     */
+    kRepeatableRead,
+  };
+
   /**
    * Options for ReadOnly transactions.
    */
@@ -100,13 +130,23 @@ class Transaction {
 
     explicit ReadWriteOptions(ReadLockMode read_lock_mode);
 
+    explicit ReadWriteOptions(IsolationLevel isolation_level)
+        : isolation_level_(isolation_level) {}
+
     // A tag used for collecting statistics about the transaction.
     ReadWriteOptions& WithTag(absl::optional<std::string> tag);
 
+    // Sets the isolation level for the transaction. This controls how the
+    // transaction interacts with other concurrent transactions, primarily
+    // regarding data consistency for reads and writes.
+    // See `IsolationLevel` enum for possible values.
+    ReadWriteOptions& WithIsolationLevel(IsolationLevel isolation_level);
+
    private:
     friend Transaction;
     google::spanner::v1::TransactionOptions_ReadWrite rw_opts_;
     absl::optional<std::string> tag_;
+    IsolationLevel isolation_level_ = IsolationLevel::kUnspecified;
   };
 
   /**
diff --git a/google/cloud/spanner/transaction_test.cc b/google/cloud/spanner/transaction_test.cc
index ac7a3b46483b5..c57f2fc7fcc55 100644
--- a/google/cloud/spanner/transaction_test.cc
+++ b/google/cloud/spanner/transaction_test.cc
@@ -14,6 +14,8 @@
 
 #include "google/cloud/spanner/transaction.h"
 #include "google/cloud/spanner/internal/session.h"
+#include "google/cloud/spanner/options.h"
+#include "google/cloud/options.h"
 #include <gmock/gmock.h>
 
 namespace google {
@@ -169,6 +171,36 @@ TEST(Transaction, MultiplexedPreviousTransactionId) {
       });
 }
 
+TEST(Transaction, IsolationLevelPrecedence) {
+  internal::OptionsSpan span(Options{}.set<TransactionIsolationLevelOption>(
+      Transaction::IsolationLevel::kSerializable));
+
+  // Case 1: Per-call overrides client default
+  auto opts = Transaction::ReadWriteOptions().WithIsolationLevel(
+      Transaction::IsolationLevel::kRepeatableRead);
+  Transaction txn = MakeReadWriteTransaction(opts);
+  spanner_internal::Visit(
+      txn, [](spanner_internal::SessionHolder&,
+              StatusOr<google::spanner::v1::TransactionSelector>& s,
+              spanner_internal::TransactionContext const&) {
+        EXPECT_EQ(s->begin().isolation_level(),
+                  google::spanner::v1::TransactionOptions::REPEATABLE_READ);
+        return 0;
+      });
+
+  // Case 2: Fallback to client default
+  auto opts_default = Transaction::ReadWriteOptions();
+  Transaction txn_default = MakeReadWriteTransaction(opts_default);
+  spanner_internal::Visit(
+      txn_default, [](spanner_internal::SessionHolder&,
+                      StatusOr<google::spanner::v1::TransactionSelector>& s,
+                      spanner_internal::TransactionContext const&) {
+        EXPECT_EQ(s->begin().isolation_level(),
+                  google::spanner::v1::TransactionOptions::SERIALIZABLE);
+        return 0;
+      });
+}
+
 TEST(Transaction, ReadWriteOptionsWithTag) {
   auto opts = Transaction::ReadWriteOptions().WithTag("test-tag");
   Transaction txn = MakeReadWriteTransaction(opts);