diff --git a/examples/platforms/simulation/diag.c b/examples/platforms/simulation/diag.c
index 5b1065c26..dcd9f1d9f 100644
--- a/examples/platforms/simulation/diag.c
+++ b/examples/platforms/simulation/diag.c
@@ -168,4 +168,12 @@ otError otPlatDiagRadioTransmitCarrier(otInstance *aInstance, bool aEnable)
 
     return OT_ERROR_NONE;
 }
+
+otError otPlatDiagRadioTransmitStream(otInstance *aInstance, bool aEnable)
+{
+    OT_UNUSED_VARIABLE(aInstance);
+    OT_UNUSED_VARIABLE(aEnable);
+
+    return OT_ERROR_NONE;
+}
 #endif // OPENTHREAD_CONFIG_DIAG_ENABLE
diff --git a/include/openthread/instance.h b/include/openthread/instance.h
index 257ae4a0e..b6bb24855 100644
--- a/include/openthread/instance.h
+++ b/include/openthread/instance.h
@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (312)
+#define OPENTHREAD_API_VERSION (313)
 
 /**
  * @addtogroup api-instance
diff --git a/include/openthread/platform/diag.h b/include/openthread/platform/diag.h
index df889d277..6f65f8234 100644
--- a/include/openthread/platform/diag.h
+++ b/include/openthread/platform/diag.h
@@ -260,6 +260,19 @@ otError otPlatDiagRadioRawPowerSettingEnable(otInstance *aInstance, bool aEnable
  */
 otError otPlatDiagRadioTransmitCarrier(otInstance *aInstance, bool aEnable);
 
+/**
+ * Start/stop the platform layer to transmit stream of characters.
+ *
+ * @param[in]  aInstance The OpenThread instance structure.
+ * @param[in]  aEnable   TRUE to enable or FALSE to disable the platform layer to transmit stream.
+ *
+ * @retval OT_ERROR_NONE             Successfully enabled/disabled.
+ * @retval OT_ERROR_INVALID_STATE    The radio was not in the Receive state.
+ * @retval OT_ERROR_NOT_IMPLEMENTED  This function is not implemented.
+ *
+ */
+otError otPlatDiagRadioTransmitStream(otInstance *aInstance, bool aEnable);
+
 /**
  * Get the power settings for the given channel.
  *
diff --git a/src/core/diags/README.md b/src/core/diags/README.md
index b137b14c7..2ddb658ac 100644
--- a/src/core/diags/README.md
+++ b/src/core/diags/README.md
@@ -10,6 +10,7 @@ The diagnostics module supports common diagnostics features that are listed belo
 - [diag start](#diag-start)
 - [diag channel](#diag-channel)
 - [diag cw](#diag-cw-start)
+- [diag stream](#diag-stream-start)
 - [diag power](#diag-power)
 - [diag powersettings](#diag-powersettings)
 - [diag send](#diag-send-packets-length)
@@ -76,6 +77,24 @@ Stop transmitting continuous carrier wave.
 Done
 ```
 
+### diag stream start
+
+Start transmitting a stream of characters.
+
+```bash
+> diag stream start
+Done
+```
+
+### diag stream stop
+
+Stop transmitting a stream of characters.
+
+```bash
+> diag stream stop
+Done
+```
+
 ### diag power
 
 Get the tx power value(dBm) for diagnostics module.
diff --git a/src/core/diags/factory_diags.cpp b/src/core/diags/factory_diags.cpp
index 542b493e7..5eed151f3 100644
--- a/src/core/diags/factory_diags.cpp
+++ b/src/core/diags/factory_diags.cpp
@@ -79,6 +79,7 @@ const struct Diags::Command Diags::sCommands[] = {
     {"rawpowersetting", &Diags::ProcessRawPowerSetting},
     {"start", &Diags::ProcessStart},
     {"stop", &Diags::ProcessStop},
+    {"stream", &Diags::ProcessStream},
 };
 
 Diags::Diags(Instance &aInstance)
@@ -196,6 +197,7 @@ const struct Diags::Command Diags::sCommands[] = {
     {"start", &Diags::ProcessStart},
     {"stats", &Diags::ProcessStats},
     {"stop", &Diags::ProcessStop},
+    {"stream", &Diags::ProcessStream},
 };
 
 Diags::Diags(Instance &aInstance)
@@ -573,6 +575,27 @@ exit:
     return error;
 }
 
+Error Diags::ProcessStream(uint8_t aArgsLength, char *aArgs[], char *aOutput, size_t aOutputMaxLen)
+{
+    Error error = kErrorInvalidArgs;
+
+    VerifyOrExit(otPlatDiagModeGet(), error = kErrorInvalidState);
+    VerifyOrExit(aArgsLength > 0, error = kErrorInvalidArgs);
+
+    if (strcmp(aArgs[0], "start") == 0)
+    {
+        error = otPlatDiagRadioTransmitStream(&GetInstance(), true);
+    }
+    else if (strcmp(aArgs[0], "stop") == 0)
+    {
+        error = otPlatDiagRadioTransmitStream(&GetInstance(), false);
+    }
+
+exit:
+    AppendErrorResult(error, aOutput, aOutputMaxLen);
+    return error;
+}
+
 Error Diags::GetPowerSettings(uint8_t aChannel, PowerSettings &aPowerSettings)
 {
     aPowerSettings.mRawPowerSetting.mLength = RawPowerSetting::kMaxDataSize;
@@ -939,6 +962,14 @@ OT_TOOL_WEAK otError otPlatDiagRadioTransmitCarrier(otInstance *aInstance, bool
     return OT_ERROR_NOT_IMPLEMENTED;
 }
 
+OT_TOOL_WEAK otError otPlatDiagRadioTransmitStream(otInstance *aInstance, bool aEnable)
+{
+    OT_UNUSED_VARIABLE(aInstance);
+    OT_UNUSED_VARIABLE(aEnable);
+
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 OT_TOOL_WEAK otError otPlatDiagRadioGetPowerSettings(otInstance *aInstance,
                                                      uint8_t     aChannel,
                                                      int16_t    *aTargetPower,
diff --git a/src/core/diags/factory_diags.hpp b/src/core/diags/factory_diags.hpp
index 9115e5150..d7ac648f9 100644
--- a/src/core/diags/factory_diags.hpp
+++ b/src/core/diags/factory_diags.hpp
@@ -193,6 +193,7 @@ private:
     Error ProcessStart(uint8_t aArgsLength, char *aArgs[], char *aOutput, size_t aOutputMaxLen);
     Error ProcessStats(uint8_t aArgsLength, char *aArgs[], char *aOutput, size_t aOutputMaxLen);
     Error ProcessStop(uint8_t aArgsLength, char *aArgs[], char *aOutput, size_t aOutputMaxLen);
+    Error ProcessStream(uint8_t aArgsLength, char *aArgs[], char *aOutput, size_t aOutputMaxLen);
 #if OPENTHREAD_RADIO && !OPENTHREAD_RADIO_CLI
     Error ProcessEcho(uint8_t aArgsLength, char *aArgs[], char *aOutput, size_t aOutputMaxLen);
 #endif
diff --git a/src/posix/platform/radio.cpp b/src/posix/platform/radio.cpp
index 0134e28d2..98f580a6f 100644
--- a/src/posix/platform/radio.cpp
+++ b/src/posix/platform/radio.cpp
@@ -735,6 +735,16 @@ exit:
     return error;
 }
 
+otError otPlatDiagRadioTransmitStream(otInstance *aInstance, bool aEnable)
+{
+    OT_UNUSED_VARIABLE(aInstance);
+
+    char cmd[OPENTHREAD_CONFIG_DIAG_CMD_LINE_BUFFER_SIZE];
+
+    snprintf(cmd, sizeof(cmd), "stream %s", aEnable ? "start" : "stop");
+    return sRadioSpinel.PlatDiagProcess(cmd, nullptr, 0);
+}
+
 void otPlatDiagRadioReceived(otInstance *aInstance, otRadioFrame *aFrame, otError aError)
 {
     OT_UNUSED_VARIABLE(aInstance);
diff --git a/tests/scripts/expect/cli-diags.exp b/tests/scripts/expect/cli-diags.exp
index ab4986f8b..e5d2fbfab 100755
--- a/tests/scripts/expect/cli-diags.exp
+++ b/tests/scripts/expect/cli-diags.exp
@@ -181,6 +181,12 @@ expect_line "Done"
 send "diag cw stop\n"
 expect_line "Done"
 
+send "diag stream start\n"
+expect_line "Done"
+
+send "diag stream stop\n"
+expect_line "Done"
+
 send "diag rawpowersetting 112233\n"
 expect_line "Done"