diff --git a/tools/otci/otci/command_handlers.py b/tools/otci/otci/command_handlers.py
index ee0b5f3eb..c8982f36f 100644
--- a/tools/otci/otci/command_handlers.py
+++ b/tools/otci/otci/command_handlers.py
@@ -107,6 +107,7 @@ class OtCliCommandRunner(OTCommandHandler):
         return repr(self.__otcli)
 
     def execute_command(self, cmd, timeout=10) -> List[str]:
+        assert not self.__should_close.is_set(), "OT CLI is already closed."
         self.__otcli.writeline(cmd)
 
         if cmd in ('reset', 'factoryreset'):
@@ -140,6 +141,7 @@ class OtCliCommandRunner(OTCommandHandler):
     def close(self):
         self.__should_close.set()
         self.__otcli.close()
+        self.__otcli_reader.join()
 
     def set_line_read_callback(self, callback: Optional[Callable[[str], Any]]):
         self.__line_read_callback = callback
@@ -181,10 +183,20 @@ class OtCliCommandRunner(OTCommandHandler):
         return output
 
     def __otcli_read_routine(self):
-        while not self.__should_close.isSet():
-            line = self.__otcli.readline()
+        while not self.__should_close.is_set():
+            try:
+                line = self.__otcli.readline()
+            except Exception:
+                if self.__should_close.is_set():
+                    break
+                else:
+                    raise
+
             logging.debug('%s: %r', self.__otcli, line)
 
+            if line is None:
+                break
+
             if line.startswith('> '):
                 line = line[2:]
 
diff --git a/tools/otci/otci/connectors.py b/tools/otci/otci/connectors.py
index 1785f111f..7d8a34241 100644
--- a/tools/otci/otci/connectors.py
+++ b/tools/otci/otci/connectors.py
@@ -30,6 +30,7 @@ import logging
 import subprocess
 import time
 from abc import abstractmethod
+from typing import Optional
 
 
 class OtCliHandler:
@@ -142,14 +143,25 @@ class OtCliSerial(OtCliHandler):
         self.__baudrate = baudrate
 
         import serial
-        self.__serial = serial.Serial(self.__dev, self.__baudrate, timeout=None, exclusive=True)
+        self.__serial = serial.Serial(self.__dev, self.__baudrate, timeout=0.1, exclusive=True)
+        self.__linebuffer = b''
 
     def __repr__(self):
         return self.__dev
 
-    def readline(self) -> str:
-        line = self.__serial.readline().decode('utf-8').rstrip('\r\n')
-        return line
+    def readline(self) -> Optional[str]:
+        while self.__serial.is_open:
+            line = self.__serial.readline()
+
+            if not line.endswith(b'\n'):
+                self.__linebuffer += line
+            else:
+                line = self.__linebuffer + line
+                self.__linebuffer = b''
+
+                return line.decode('utf-8').rstrip('\r\n')
+
+        return None
 
     def writeline(self, s: str):
         self.__serial.write((s + '\n').encode('utf-8'))