diff --git a/examples/platforms/cc1352/uart.c b/examples/platforms/cc1352/uart.c
index 99881eb9f..34cd86504 100644
--- a/examples/platforms/cc1352/uart.c
+++ b/examples/platforms/cc1352/uart.c
@@ -192,10 +192,7 @@ static void processReceive(void)
     }
 }
 
-/**
- * @brief process the transmit side of the buffers
- */
-static void processTransmit(void)
+otError otPlatUartFlush(void)
 {
     otEXPECT(sSendBuffer != NULL);
 
@@ -207,10 +204,20 @@ static void processTransmit(void)
 
     sSendBuffer = NULL;
     sSendLen    = 0;
-    otPlatUartSendDone();
+
+    return OT_ERROR_NONE;
 
 exit:
-    return;
+    return OT_ERROR_INVALID_STATE;
+}
+
+/**
+ * @brief process the transmit side of the buffers
+ */
+static void processTransmit(void)
+{
+    otPlatUartFlush();
+    otPlatUartSendDone();
 }
 
 /**
diff --git a/examples/platforms/cc2538/uart.c b/examples/platforms/cc2538/uart.c
index d709b840f..992af65d5 100644
--- a/examples/platforms/cc2538/uart.c
+++ b/examples/platforms/cc2538/uart.c
@@ -179,7 +179,7 @@ void processReceive(void)
     }
 }
 
-void processTransmit(void)
+otError otPlatUartFlush(void)
 {
     otEXPECT(sTransmitBuffer != NULL);
 
@@ -192,10 +192,16 @@ void processTransmit(void)
     }
 
     sTransmitBuffer = NULL;
-    otPlatUartSendDone();
+    return OT_ERROR_NONE;
 
 exit:
-    return;
+    return OT_ERROR_INVALID_STATE;
+}
+
+void processTransmit(void)
+{
+    otPlatUartFlush();
+    otPlatUartSendDone();
 }
 
 void cc2538UartProcess(void)
diff --git a/examples/platforms/cc2650/uart.c b/examples/platforms/cc2650/uart.c
index fe8b38840..d6f048325 100644
--- a/examples/platforms/cc2650/uart.c
+++ b/examples/platforms/cc2650/uart.c
@@ -147,10 +147,7 @@ static void processReceive(void)
     }
 }
 
-/**
- * @brief process the transmit side of the buffers
- */
-static void processTransmit(void)
+otError otPlatUartFlush(void)
 {
     otEXPECT(sSendBuffer != NULL);
 
@@ -162,10 +159,20 @@ static void processTransmit(void)
 
     sSendBuffer = NULL;
     sSendLen    = 0;
-    otPlatUartSendDone();
+
+    return OT_ERROR_NONE;
 
 exit:
-    return;
+    return OT_ERROR_INVALID_STATE;
+}
+
+/**
+ * @brief process the transmit side of the buffers
+ */
+static void processTransmit(void)
+{
+    otPlatUartFlush();
+    otPlatUartSendDone();
 }
 
 /**
diff --git a/examples/platforms/cc2652/uart.c b/examples/platforms/cc2652/uart.c
index 493aa0870..b342a5275 100644
--- a/examples/platforms/cc2652/uart.c
+++ b/examples/platforms/cc2652/uart.c
@@ -192,10 +192,7 @@ static void processReceive(void)
     }
 }
 
-/**
- * @brief process the transmit side of the buffers
- */
-static void processTransmit(void)
+otError otPlatUartFlush(void)
 {
     otEXPECT(sSendBuffer != NULL);
 
@@ -207,10 +204,20 @@ static void processTransmit(void)
 
     sSendBuffer = NULL;
     sSendLen    = 0;
-    otPlatUartSendDone();
+
+    return OT_ERROR_NONE;
 
 exit:
-    return;
+    return OT_ERROR_INVALID_STATE;
+}
+
+/**
+ * @brief process the transmit side of the buffers
+ */
+static void processTransmit(void)
+{
+    otPlatUartFlush();
+    otPlatUartSendDone();
 }
 
 /**
diff --git a/examples/platforms/efr32mg12/uart.c b/examples/platforms/efr32mg12/uart.c
index 36858acd8..5576d469b 100644
--- a/examples/platforms/efr32mg12/uart.c
+++ b/examples/platforms/efr32mg12/uart.c
@@ -253,6 +253,11 @@ static void processReceive(void)
     CORE_EXIT_NVIC();
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 static void processTransmit(void)
 {
     if (sTransmitBuffer != NULL && sTransmitLength == 0)
diff --git a/examples/platforms/efr32mg21/uart.c b/examples/platforms/efr32mg21/uart.c
index cffafc00c..98c67ee84 100644
--- a/examples/platforms/efr32mg21/uart.c
+++ b/examples/platforms/efr32mg21/uart.c
@@ -250,6 +250,11 @@ static void processReceive(void)
     CORE_EXIT_NVIC();
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 static void processTransmit(void)
 {
     if (sTransmitBuffer != NULL && sTransmitLength == 0)
diff --git a/examples/platforms/gp712/uart-posix.c b/examples/platforms/gp712/uart-posix.c
index 7f80c203c..fee2e217c 100644
--- a/examples/platforms/gp712/uart-posix.c
+++ b/examples/platforms/gp712/uart-posix.c
@@ -207,6 +207,11 @@ exit:
     return error;
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 void platformUartProcess(void)
 {
     ssize_t       rval;
diff --git a/examples/platforms/gp712/uart-socket.c b/examples/platforms/gp712/uart-socket.c
index 36b9f1c43..754d56eb3 100644
--- a/examples/platforms/gp712/uart-socket.c
+++ b/examples/platforms/gp712/uart-socket.c
@@ -149,6 +149,11 @@ otError otPlatUartSend(const uint8_t *aBuf, uint16_t aBufLength)
     return error;
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 void platformUartInit(void)
 {
 }
diff --git a/examples/platforms/kw41z/uart.c b/examples/platforms/kw41z/uart.c
index 6f2b671b1..226e04e16 100644
--- a/examples/platforms/kw41z/uart.c
+++ b/examples/platforms/kw41z/uart.c
@@ -120,6 +120,11 @@ exit:
     return error;
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 static void processTransmit(void)
 {
     if (sTransmitBuffer && sTransmitDone)
diff --git a/examples/platforms/nrf52811/uart.c b/examples/platforms/nrf52811/uart.c
index c56bae881..73383162d 100644
--- a/examples/platforms/nrf52811/uart.c
+++ b/examples/platforms/nrf52811/uart.c
@@ -119,6 +119,11 @@ exit:
     return;
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 /**
  * Function for notifying application about transmission being done.
  */
diff --git a/examples/platforms/nrf52840/uart.c b/examples/platforms/nrf52840/uart.c
index d871b402e..c9b1ff59a 100644
--- a/examples/platforms/nrf52840/uart.c
+++ b/examples/platforms/nrf52840/uart.c
@@ -119,6 +119,11 @@ exit:
     return;
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 /**
  * Function for notifying application about transmission being done.
  */
diff --git a/examples/platforms/nrf52840/usb-cdc-uart.c b/examples/platforms/nrf52840/usb-cdc-uart.c
index 8c325b1ed..48e0ce361 100644
--- a/examples/platforms/nrf52840/usb-cdc-uart.c
+++ b/examples/platforms/nrf52840/usb-cdc-uart.c
@@ -362,4 +362,9 @@ exit:
     return error;
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 #endif // USB_CDC_AS_SERIAL_TRANSPORT == 1
diff --git a/examples/platforms/posix/sim/platform-sim.c b/examples/platforms/posix/sim/platform-sim.c
index 6d38e0824..87936c651 100644
--- a/examples/platforms/posix/sim/platform-sim.c
+++ b/examples/platforms/posix/sim/platform-sim.c
@@ -165,6 +165,11 @@ otError otPlatUartSend(const uint8_t *aData, uint16_t aLength)
 
     return error;
 }
+
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
 #endif // OPENTHREAD_POSIX_VIRTUAL_TIME_UART
 
 static void socket_init(void)
diff --git a/examples/platforms/posix/uart-posix.c b/examples/platforms/posix/uart-posix.c
index 391b4b118..af375dcae 100644
--- a/examples/platforms/posix/uart-posix.c
+++ b/examples/platforms/posix/uart-posix.c
@@ -222,6 +222,11 @@ void platformUartUpdateFdSet(fd_set *aReadFdSet, fd_set *aWriteFdSet, fd_set *aE
     }
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 void platformUartProcess(void)
 {
     ssize_t       rval;
diff --git a/examples/platforms/qpg6095/uart.c b/examples/platforms/qpg6095/uart.c
index eaf027dc6..c5336a260 100644
--- a/examples/platforms/qpg6095/uart.c
+++ b/examples/platforms/qpg6095/uart.c
@@ -57,6 +57,11 @@ otError otPlatUartDisable(void)
     return error;
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 otError otPlatUartSend(const uint8_t *aBuf, uint16_t aBufLength)
 {
     otError error = OT_ERROR_NONE;
diff --git a/examples/platforms/samr21/uart.c b/examples/platforms/samr21/uart.c
index 3c7da8547..8f8aa1eaa 100644
--- a/examples/platforms/samr21/uart.c
+++ b/examples/platforms/samr21/uart.c
@@ -97,6 +97,11 @@ static void processReceive(void)
     }
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 static void processTransmit(void)
 {
     if (sTransmitDone)
diff --git a/include/openthread/platform/uart.h b/include/openthread/platform/uart.h
index c6a89b460..6366dfefc 100644
--- a/include/openthread/platform/uart.h
+++ b/include/openthread/platform/uart.h
@@ -83,6 +83,19 @@ otError otPlatUartDisable(void);
  */
 otError otPlatUartSend(const uint8_t *aBuf, uint16_t aBufLength);
 
+/**
+ * Flush the outgoing transmit buffer and wait for the data to be sent.
+ * This is called when the CLI UART interface has a full buffer but still
+ * wishes to send more data.
+ *
+ * @retval OT_ERROR_NONE                Flush succeeded, we can proceed to write more
+ *                                      data to the buffer.
+ *
+ * @retval OT_ERROR_NOT_IMPLEMENTED     Driver does not support synchronous flush.
+ * @retval OT_ERROR_INVALID_STATE       Driver has no data to flush.
+ */
+otError otPlatUartFlush(void);
+
 /**
  * The UART driver calls this method to notify OpenThread that the requested bytes have been sent.
  *
diff --git a/src/cli/cli_uart.cpp b/src/cli/cli_uart.cpp
index 9818a9d5d..097561a6e 100644
--- a/src/cli/cli_uart.cpp
+++ b/src/cli/cli_uart.cpp
@@ -237,25 +237,52 @@ otError Uart::ProcessCommand(void)
 int Uart::Output(const char *aBuf, uint16_t aBufLength)
 {
     OT_CLI_UART_OUTPUT_LOCK();
-    uint16_t remaining = kTxBufferSize - mTxLength;
-    uint16_t tail;
+    uint16_t sent = 0;
 
-    if (aBufLength > remaining)
+    while (aBufLength > 0)
     {
-        aBufLength = remaining;
+        uint16_t remaining = kTxBufferSize - mTxLength;
+        uint16_t tail;
+        uint16_t sendLength = aBufLength;
+
+        if (sendLength > remaining)
+        {
+            sendLength = remaining;
+        }
+
+        for (uint16_t i = 0; i < sendLength; i++)
+        {
+            tail            = (mTxHead + mTxLength) % kTxBufferSize;
+            mTxBuffer[tail] = *aBuf++;
+            aBufLength--;
+            mTxLength++;
+        }
+
+        Send();
+
+        sent += sendLength;
+
+        if (aBufLength > 0)
+        {
+            // More to send, so flush what's waiting now
+            otError err = otPlatUartFlush();
+
+            if (err == OT_ERROR_NONE)
+            {
+                // Flush successful, reset the pointers
+                SendDoneTask();
+            }
+            else
+            {
+                // Flush did not succeed, so abort here.
+                break;
+            }
+        }
     }
 
-    for (int i = 0; i < aBufLength; i++)
-    {
-        tail            = (mTxHead + mTxLength) % kTxBufferSize;
-        mTxBuffer[tail] = *aBuf++;
-        mTxLength++;
-    }
-
-    Send();
     OT_CLI_UART_OUTPUT_UNLOCK();
 
-    return aBufLength;
+    return sent;
 }
 
 void Uart::Send(void)
diff --git a/src/posix/platform/uart.c b/src/posix/platform/uart.c
index 93859ff82..4534ceee0 100644
--- a/src/posix/platform/uart.c
+++ b/src/posix/platform/uart.c
@@ -157,6 +157,11 @@ exit:
     return error;
 }
 
+otError otPlatUartFlush(void)
+{
+    return OT_ERROR_NOT_IMPLEMENTED;
+}
+
 void platformUartUpdateFdSet(fd_set *aReadFdSet, fd_set *aWriteFdSet, fd_set *aErrorFdSet, int *aMaxFd)
 {
     otEXPECT(sEnabled);