Justin Bieber -- Mother's Day with the Clippers ... We're Sittin' Courtside!

Like a good son should, Justin Bieber is spending his Mother's Day with his mom ... hangin' out courtside at the Clippers vs. Thunder playoff game in L.A. ... and TMZ Sports has the pic.

Sources at the game tell us ... Bieber and his mom Pattie Mallette seem like they've been having a good time -- and spent some time chattin' with other huge stars including Mark Wahlberg and Floyd Mayweather.

Bieber even wore a shirt to the game ... though, he left his sleeves at home.