diff --git a/CHANGES.md b/CHANGES.md
index a028ef6f1e..5ab9ff733d 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -5,7 +5,7 @@ Features ✨:
  - Enable url previews for notices (#2562)
 
 Improvements 🙌:
- -
+  - Add System theme option and set as default (#904) (#2387)
 
 Bugfix 🐛:
  - Url previews sometimes attached to wrong message (#2561)
diff --git a/vector/src/main/java/im/vector/app/features/themes/ThemeUtils.kt b/vector/src/main/java/im/vector/app/features/themes/ThemeUtils.kt
index bba6b9c253..172303cff7 100644
--- a/vector/src/main/java/im/vector/app/features/themes/ThemeUtils.kt
+++ b/vector/src/main/java/im/vector/app/features/themes/ThemeUtils.kt
@@ -18,6 +18,8 @@ package im.vector.app.features.themes
 
 import android.app.Activity
 import android.content.Context
+import android.content.res.Configuration
+import android.content.res.Resources
 import android.graphics.drawable.Drawable
 import android.util.TypedValue
 import android.view.Menu
@@ -39,6 +41,7 @@ object ThemeUtils {
     const val APPLICATION_THEME_KEY = "APPLICATION_THEME_KEY"
 
     // the theme possible values
+    private const val SYSTEM_THEME_VALUE = "system"
     private const val THEME_DARK_VALUE = "dark"
     private const val THEME_LIGHT_VALUE = "light"
     private const val THEME_BLACK_VALUE = "black"
@@ -54,13 +57,11 @@ object ThemeUtils {
     }
 
     /**
-     * @return true if current theme is Light or Status
+     * @return true if current theme is Light/Status or current theme is System and system theme is light
      */
     fun isLightTheme(context: Context): Boolean {
-        return when (getApplicationTheme(context)) {
-            THEME_LIGHT_VALUE -> true
-            else              -> false
-        }
+        val theme = getApplicationTheme(context)
+        return theme == THEME_LIGHT_VALUE || (theme == SYSTEM_THEME_VALUE && !isSystemDarkTheme(context.resources))
     }
 
     /**
@@ -73,11 +74,11 @@ object ThemeUtils {
         val currentTheme = this.currentTheme.get()
         return if (currentTheme == null) {
             val prefs = DefaultSharedPreferences.getInstance(context)
-            var themeFromPref = prefs.getString(APPLICATION_THEME_KEY, THEME_LIGHT_VALUE) ?: THEME_LIGHT_VALUE
+            var themeFromPref = prefs.getString(APPLICATION_THEME_KEY, SYSTEM_THEME_VALUE) ?: SYSTEM_THEME_VALUE
             if (themeFromPref == "status") {
                 // Migrate to light theme, which is the closest theme
-                themeFromPref = THEME_LIGHT_VALUE
-                prefs.edit { putString(APPLICATION_THEME_KEY, THEME_LIGHT_VALUE) }
+                themeFromPref = SYSTEM_THEME_VALUE
+                prefs.edit { putString(APPLICATION_THEME_KEY, SYSTEM_THEME_VALUE) }
             }
             this.currentTheme.set(themeFromPref)
             themeFromPref
@@ -86,6 +87,13 @@ object ThemeUtils {
         }
     }
 
+    /**
+     * @return true if system theme is dark
+     */
+    private fun isSystemDarkTheme(resources: Resources): Boolean {
+        return resources.configuration.uiMode and Configuration.UI_MODE_NIGHT_MASK == Configuration.UI_MODE_NIGHT_YES
+    }
+
     /**
      * Update the application theme
      *
@@ -94,6 +102,7 @@ object ThemeUtils {
     fun setApplicationTheme(context: Context, aTheme: String) {
         currentTheme.set(aTheme)
         when (aTheme) {
+            SYSTEM_THEME_VALUE -> context.setTheme(if(isSystemDarkTheme(context.resources)) R.style.AppTheme_Dark else R.style.AppTheme_Light)
             THEME_DARK_VALUE   -> context.setTheme(R.style.AppTheme_Dark)
             THEME_BLACK_VALUE  -> context.setTheme(R.style.AppTheme_Black)
             else               -> context.setTheme(R.style.AppTheme_Light)
@@ -110,6 +119,7 @@ object ThemeUtils {
      */
     fun setActivityTheme(activity: Activity, otherThemes: ActivityOtherThemes) {
         when (getApplicationTheme(activity)) {
+            SYSTEM_THEME_VALUE -> if(isSystemDarkTheme(activity.resources)) activity.setTheme(otherThemes.dark)
             THEME_DARK_VALUE   -> activity.setTheme(otherThemes.dark)
             THEME_BLACK_VALUE  -> activity.setTheme(otherThemes.black)
         }
diff --git a/vector/src/main/res/values/array.xml b/vector/src/main/res/values/array.xml
index e56844e2e8..2c4bc135e5 100644
--- a/vector/src/main/res/values/array.xml
+++ b/vector/src/main/res/values/array.xml
@@ -92,12 +92,14 @@
 
     <!-- Theme -->
     <string-array name="theme_entries">
+        <item>@string/system_theme</item>
         <item>@string/light_theme</item>
         <item>@string/dark_theme</item>
         <item>@string/black_them</item>
     </string-array>
 
     <string-array name="theme_values">
+        <item>system</item>
         <item>light</item>
         <item>dark</item>
         <item>black</item>
diff --git a/vector/src/main/res/values/strings.xml b/vector/src/main/res/values/strings.xml
index 76541460d2..355ac4d6d6 100644
--- a/vector/src/main/res/values/strings.xml
+++ b/vector/src/main/res/values/strings.xml
@@ -8,6 +8,7 @@
     <string name="resources_script">Latn</string>
 
     <!-- theme -->
+    <string name="system_theme">System Default</string>
     <string name="light_theme">Light Theme</string>
     <string name="dark_theme">Dark Theme</string>
     <string name="black_them">Black Theme</string>
diff --git a/vector/src/main/res/xml/vector_settings_preferences.xml b/vector/src/main/res/xml/vector_settings_preferences.xml
index 74b1f882ee..6297b89e6c 100644
--- a/vector/src/main/res/xml/vector_settings_preferences.xml
+++ b/vector/src/main/res/xml/vector_settings_preferences.xml
@@ -13,7 +13,7 @@
             app:fragment="im.vector.app.features.settings.locale.LocalePickerFragment" />
 
         <im.vector.app.core.preference.VectorListPreference
-            android:defaultValue="light"
+            android:defaultValue="system"
             android:entries="@array/theme_entries"
             android:entryValues="@array/theme_values"
             android:key="APPLICATION_THEME_KEY"