diff --git a/plugins/inputs/modbus/README.md b/plugins/inputs/modbus/README.md
index 5112b8d28..1bc4ef086 100644
--- a/plugins/inputs/modbus/README.md
+++ b/plugins/inputs/modbus/README.md
@@ -50,10 +50,13 @@ See the [CONFIGURATION.md][CONFIGURATION.md] for more details.
   # parity = "N"
   # stop_bits = 1
 
-  ## For Modbus over TCP you can choose between "TCP", "RTUoverTCP" and
-  ## "ASCIIoverTCP". The default behaviour is "TCP" for ModbusTCP controllers.
+  ## Transmission mode for Modbus packets depending on the controller type.
+  ## For Modbus over TCP you can choose between "TCP" , "RTUoverTCP" and
+  ## "ASCIIoverTCP".
   ## For Serial controllers you can choose between "RTU" and "ASCII".
-  # transmission_mode = "RTU"
+  ## By default this is set to "auto" selecting "TCP" for ModbusTCP connections
+  ## and "RTU" for serial connections.
+  # transmission_mode = "auto"
 
   ## Trace the connection to the modbus device as debug messages
   ## Note: You have to enable telegraf's debug mode to see those messages!
diff --git a/plugins/inputs/modbus/modbus.go b/plugins/inputs/modbus/modbus.go
index e1d64826e..61b4f4b38 100644
--- a/plugins/inputs/modbus/modbus.go
+++ b/plugins/inputs/modbus/modbus.go
@@ -235,6 +235,13 @@ func (m *Modbus) initClient() error {
 			return err
 		}
 		switch m.TransmissionMode {
+		case "", "auto", "TCP":
+			handler := mb.NewTCPClientHandler(host + ":" + port)
+			handler.Timeout = time.Duration(m.Timeout)
+			if m.DebugConnection {
+				handler.Logger = m
+			}
+			m.handler = handler
 		case "RTUoverTCP":
 			handler := mb.NewRTUOverTCPClientHandler(host + ":" + port)
 			handler.Timeout = time.Duration(m.Timeout)
@@ -250,12 +257,7 @@ func (m *Modbus) initClient() error {
 			}
 			m.handler = handler
 		default:
-			handler := mb.NewTCPClientHandler(host + ":" + port)
-			handler.Timeout = time.Duration(m.Timeout)
-			if m.DebugConnection {
-				handler.Logger = m
-			}
-			m.handler = handler
+			return fmt.Errorf("invalid transmission mode %q for %q", m.TransmissionMode, u.Scheme)
 		}
 	case "", "file":
 		path := filepath.Join(u.Host, u.Path)
@@ -263,7 +265,7 @@ func (m *Modbus) initClient() error {
 			return fmt.Errorf("invalid path for controller %q", m.Controller)
 		}
 		switch m.TransmissionMode {
-		case "RTU":
+		case "", "auto", "RTU":
 			handler := mb.NewRTUClientHandler(path)
 			handler.Timeout = time.Duration(m.Timeout)
 			handler.BaudRate = m.BaudRate
@@ -286,7 +288,7 @@ func (m *Modbus) initClient() error {
 			}
 			m.handler = handler
 		default:
-			return fmt.Errorf("invalid protocol '%s' - '%s' ", u.Scheme, m.TransmissionMode)
+			return fmt.Errorf("invalid transmission mode %q for %q", m.TransmissionMode, u.Scheme)
 		}
 	default:
 		return fmt.Errorf("invalid controller %q", m.Controller)
diff --git a/plugins/inputs/modbus/modbus_test.go b/plugins/inputs/modbus/modbus_test.go
index 29e8bd45c..4fe5c6f96 100644
--- a/plugins/inputs/modbus/modbus_test.go
+++ b/plugins/inputs/modbus/modbus_test.go
@@ -25,6 +25,7 @@ func TestControllers(t *testing.T) {
 	var tests = []struct {
 		name       string
 		controller string
+		mode       string
 		errmsg     string
 	}{
 		{
@@ -32,9 +33,41 @@ func TestControllers(t *testing.T) {
 			controller: "tcp://localhost:502",
 		},
 		{
-			name:       "invalid TCP host",
+			name:       "TCP mode auto",
+			controller: "tcp://localhost:502",
+			mode:       "auto",
+		},
+		{
+			name:       "TCP mode TCP",
+			controller: "tcp://localhost:502",
+			mode:       "TCP",
+		},
+		{
+			name:       "TCP mode RTUoverTCP",
+			controller: "tcp://localhost:502",
+			mode:       "RTUoverTCP",
+		},
+		{
+			name:       "TCP mode ASCIIoverTCP",
+			controller: "tcp://localhost:502",
+			mode:       "ASCIIoverTCP",
+		},
+		{
+			name:       "TCP invalid host",
 			controller: "tcp://localhost",
-			errmsg:     "initializing client failed: address localhost: missing port in address",
+			errmsg:     "address localhost: missing port in address",
+		},
+		{
+			name:       "TCP invalid mode RTU",
+			controller: "tcp://localhost:502",
+			mode:       "RTU",
+			errmsg:     "invalid transmission mode",
+		},
+		{
+			name:       "TCP invalid mode ASCII",
+			controller: "tcp://localhost:502",
+			mode:       "ASCII",
+			errmsg:     "invalid transmission mode",
 		},
 		{
 			name:       "absolute file path",
@@ -56,20 +89,41 @@ func TestControllers(t *testing.T) {
 			name:       "Windows COM-port file path",
 			controller: "file://com2",
 		},
+		{
+			name:       "serial mode auto",
+			controller: "file:///dev/ttyUSB0",
+			mode:       "auto",
+		},
+		{
+			name:       "serial mode RTU",
+			controller: "file:///dev/ttyUSB0",
+			mode:       "RTU",
+		},
+		{
+			name:       "serial mode ASCII",
+			controller: "file:///dev/ttyUSB0",
+			mode:       "ASCII",
+		},
 		{
 			name:       "empty file path",
 			controller: "file://",
-			errmsg:     "initializing client failed: invalid path for controller",
+			errmsg:     "invalid path for controller",
 		},
 		{
 			name:       "empty controller",
 			controller: "",
-			errmsg:     "initializing client failed: invalid path for controller",
+			errmsg:     "invalid path for controller",
 		},
 		{
 			name:       "invalid scheme",
 			controller: "foo://bar",
-			errmsg:     "initializing client failed: invalid controller",
+			errmsg:     "invalid controller",
+		},
+		{
+			name:       "serial invalid mode TCP",
+			controller: "file:///dev/ttyUSB0",
+			mode:       "TCP",
+			errmsg:     "invalid transmission mode",
 		},
 	}
 
@@ -78,7 +132,7 @@ func TestControllers(t *testing.T) {
 			plugin := Modbus{
 				Name:             "dummy",
 				Controller:       tt.controller,
-				TransmissionMode: "RTU",
+				TransmissionMode: tt.mode,
 				Log:              testutil.Logger{},
 			}
 			err := plugin.Init()
diff --git a/plugins/inputs/modbus/sample_general_begin.conf b/plugins/inputs/modbus/sample_general_begin.conf
index 54bc7e7d9..e88109f91 100644
--- a/plugins/inputs/modbus/sample_general_begin.conf
+++ b/plugins/inputs/modbus/sample_general_begin.conf
@@ -33,10 +33,13 @@
   # parity = "N"
   # stop_bits = 1
 
-  ## For Modbus over TCP you can choose between "TCP", "RTUoverTCP" and
-  ## "ASCIIoverTCP". The default behaviour is "TCP" for ModbusTCP controllers.
+  ## Transmission mode for Modbus packets depending on the controller type.
+  ## For Modbus over TCP you can choose between "TCP" , "RTUoverTCP" and
+  ## "ASCIIoverTCP".
   ## For Serial controllers you can choose between "RTU" and "ASCII".
-  # transmission_mode = "RTU"
+  ## By default this is set to "auto" selecting "TCP" for ModbusTCP connections
+  ## and "RTU" for serial connections.
+  # transmission_mode = "auto"
 
   ## Trace the connection to the modbus device as debug messages
   ## Note: You have to enable telegraf's debug mode to see those messages!