In Android we have, Settings.Secure.ANDROID_ID. I do not know the iOS equivalent.
Is there a flutter plugin or a way to get a unique device id for both Android
UPDATE: Because this thread has a very high Google ranking I wanted to mention here that the device_id package has been discontinued. The package platform_device_id looks like it works about the same way though, and has very recent activity. We're switching over to that after seeing errors in the log after iOS crashes which point to the old package.