diff --git a/pkg/config/config.go b/pkg/config/config.go
index 5a6a0cf9..e1544e62 100644
--- a/pkg/config/config.go
+++ b/pkg/config/config.go
@@ -21,6 +21,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"runtime"
 
 	"github.com/ProtonMail/go-appdir"
 	"github.com/hashicorp/go-multierror"
@@ -247,3 +248,17 @@ func (c *Config) GetDefaultIMAPPort() int {
 func (c *Config) GetDefaultSMTPPort() int {
 	return 1025
 }
+
+// getAPIOS returns actual operating system.
+func (c *Config) getAPIOS() string {
+	switch os := runtime.GOOS; os {
+	case "darwin": // nolint: goconst
+		return "macOS"
+	case "linux":
+		return "Linux"
+	case "windows":
+		return "Windows"
+	}
+
+	return "Linux"
+}
diff --git a/pkg/config/pmapi_noprod.go b/pkg/config/pmapi_noprod.go
index 653d1afb..ec1c093d 100644
--- a/pkg/config/pmapi_noprod.go
+++ b/pkg/config/pmapi_noprod.go
@@ -29,7 +29,7 @@ import (
 
 func (c *Config) GetAPIConfig() *pmapi.ClientConfig {
 	return &pmapi.ClientConfig{
-		AppVersion: strings.Title(c.appName) + "_" + c.version,
+		AppVersion: c.getAPIOS() + strings.Title(c.appName) + "_" + c.version,
 		ClientID:   c.appName,
 	}
 }
diff --git a/pkg/config/pmapi_prod.go b/pkg/config/pmapi_prod.go
index 38cd22a6..fde1274c 100644
--- a/pkg/config/pmapi_prod.go
+++ b/pkg/config/pmapi_prod.go
@@ -31,7 +31,7 @@ import (
 
 func (c *Config) GetAPIConfig() *pmapi.ClientConfig {
 	return &pmapi.ClientConfig{
-		AppVersion:        strings.Title(c.appName) + "_" + c.version,
+		AppVersion:        c.getAPIOS() + strings.Title(c.appName) + "_" + c.version,
 		ClientID:          c.appName,
 		Timeout:           25 * time.Minute, // Overall request timeout (~25MB / 25 mins => ~16kB/s, should be reasonable).
 		FirstReadTimeout:  30 * time.Second, // 30s to match 30s response header timeout.