Bobby Brown Belts Out 'My Girl' to Celebrate New Baby

Bobby Brown doesn't sit around hospital rooms anymore when he has a baby -- instead he celebrates by singing with random bands in Bev Hills ... and it's actually pretty awesome.

We got this video of Bobby doing "My Girl" Thursday night at Caffe Roma ... shortly after his wife, Alicia, gave birth to their third child ... a 10 pound baby girl.

Witnesses tell us Bobby said he was driving around town, taking a breather after the birth. When he heard the band jamming, he decided to stop and join 'em.