diff --git a/CHANGELOG.md b/CHANGELOG.md
index e5563813..af5face7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 ## [Unreleased]
 
+### Bug fixes
+
+- Fixed a bug: the timeout in IProtoBegin was being specified in milliseconds (as a `long`), but
+  the [protocol](https://www.tarantool.io/ru/doc/latest/reference/internals/iproto/streams/#iproto-begin)
+  requires the timeout to be passed in seconds (as a `double`).
+
 ## [1.7.2] - 2026-09-25
 
 ### Client
diff --git a/tarantool-core/src/main/java/io/tarantool/core/protocol/requests/IProtoBegin.java b/tarantool-core/src/main/java/io/tarantool/core/protocol/requests/IProtoBegin.java
index 76ce959c..d5f834ef 100644
--- a/tarantool-core/src/main/java/io/tarantool/core/protocol/requests/IProtoBegin.java
+++ b/tarantool-core/src/main/java/io/tarantool/core/protocol/requests/IProtoBegin.java
@@ -27,14 +27,14 @@
 public class IProtoBegin extends IProtoBaseRequest {
 
   // body fields
-  private final long timeout;
+  private final double timeoutS;
   private final TransactionIsolationLevel level;
 
-  public IProtoBegin(Long streamId, long timeout, TransactionIsolationLevel level) {
-    if (timeout <= 0) {
-      throw new IllegalArgumentException("timeout should be greater than 0");
+  public IProtoBegin(Long streamId, long timeoutMs, TransactionIsolationLevel level) {
+    if (timeoutMs <= 0) {
+      throw new IllegalArgumentException("timeoutMs should be greater than 0");
     }
-    this.timeout = timeout;
+    this.timeoutS = timeoutMs / 1_000d;
     this.level = level;
     this.setStreamId(streamId);
   }
@@ -55,7 +55,7 @@ public byte[] getPacket(MessageBufferPacker packer) throws IOException {
     }
 
     packer.addPayload(RAW_IPROTO_TIMEOUT); // key
-    packer.packDouble(timeout); // value
+    packer.packDouble(timeoutS); // value
 
     if (!level.equals(TransactionIsolationLevel.DEFAULT)) {
       packer.addPayload(RAW_IPROTO_TXN_ISOLATION); // key
@@ -68,7 +68,7 @@ public byte[] getPacket(MessageBufferPacker packer) throws IOException {
   @Override
   public MapValue getBody() {
     Map<Value, Value> map = new HashMap<>();
-    map.put(MP_IPROTO_TIMEOUT, ValueFactory.newInteger(timeout));
+    map.put(MP_IPROTO_TIMEOUT, ValueFactory.newFloat(timeoutS));
 
     if (!level.equals(TransactionIsolationLevel.DEFAULT)) {
       map.put(MP_IPROTO_TXN_ISOLATION, ValueFactory.newInteger(level.ordinal()));
@@ -94,8 +94,8 @@ public String toString() {
           .append("IProtoBegin(syncId = ")
           .append(getSyncId())
           .append(", timeout = ")
-          .append(timeout)
-          .append(", level = ")
+          .append(timeoutS)
+          .append("s, level = ")
           .append(level)
           .append(")");
     }
diff --git a/tarantool-core/src/test/java/io/tarantool/core/unit/requests/RequestTest.java b/tarantool-core/src/test/java/io/tarantool/core/unit/requests/RequestTest.java
index 70877034..c9499019 100644
--- a/tarantool-core/src/test/java/io/tarantool/core/unit/requests/RequestTest.java
+++ b/tarantool-core/src/test/java/io/tarantool/core/unit/requests/RequestTest.java
@@ -60,14 +60,15 @@ public void testRequestsToString() {
 
     // begin
     final Long streamId = 1L;
-    long timeout = 1L;
-    IProtoRequest begin = new IProtoBegin(streamId, timeout, TransactionIsolationLevel.DEFAULT);
+    long timeoutMs = 1L;
+    double timeoutS = timeoutMs / 1_000d;
+    IProtoRequest begin = new IProtoBegin(streamId, timeoutMs, TransactionIsolationLevel.DEFAULT);
     assertEquals(
         "IProtoBegin(syncId = "
             + begin.getSyncId()
             + ", timeout = "
-            + timeout
-            + ", level = "
+            + timeoutS
+            + "s, level = "
             + TransactionIsolationLevel.DEFAULT
             + ")",
         begin.toString());