Janet Jackson rocked the American Idol stage last night ... revealing a tight and toned body, and sexy short 'do, in front of thousands at the Nokia Theater.

Miss Jackson performed three songs ... including one of her biggest hits, Nasty. We have a feeling that one was for Simon!

After watching her work the stage, there's no question that the 44-year-old is still at the top of her game. Watch and learn future Idols!