Steelers star JuJu Smith-Schuster posted video appearing to show him in the driver's seat of a BMW, hitting 104 MPH ... less than 2 weeks after suffering a severe head injury.

The Pittsburgh WR posted the video on to his Instagram stories late Tuesday night ... seemingly wanting to show off his fancy ride and the Travis Scott banger that was playing over his car's speakers.

The problem for JuJu? He suffered a serious concussion in the Steelers' game against the Browns on Nov. 14 ... and speeding down the highway with a phone locked in his hand ain't exactly the smartest call.

Hell, even if he DIDN'T have a concussion, ya shouldn't IG and drive (especially at 104 mph).

JuJu has since removed the video from his IG page.

Of course, this ain't the first time a young NFL superstar has tried to flex behind the wheel at breakneck speed ... back in March, Lamar Jackson did the SAME THING!!!

The Ravens QB -- who topped out at 105 MPH -- eventually apologized for the dumb move ... and went on to become the best player in the NFL this season.

Seems JuJu should learn a thing or two from the league's (likely) MVP.