TMZ has posted some pictures that allegedly show JUSTIN BIEBER smoking pot in a California hotel room.  They were taken last Wednesday.  (-- CLICKHERE.  He's definitely smoking SOMETHING.)

--A source says Justin's buddy, 19-year-old rapper LIL TWIST, was there, too.  In fact, it was Twist and his brother who were rolling all the blunts.

--Justin hasn't copped to toking, but he did Tweet this after the pictures hit the web . . . quote, "Everyday growing and learning.  Trying to be better.  u get knocked down, u get up.

--"Back on tour tomorrow.  Ready to see u all smile.  Time to do what I'm supposed to be doing . . . performing."