Maksim Chmerkovskiy -- I'm Coming Back To 'DWTS'!

Maksim Chmerkovskiy has really big disco balls ... 'cause he's not only returning to "Dancing With the Stars" -- but he's going back with a bigger job ... TMZ has learned.

According to our ballroom sources, Maksim will make a surprise appearance Monday on "DWTS' ... as a guest judge.

We're told the show's former dance pro will be sitting right next to Bruno, Len and Carrie Ann during the semi-final performances.

Maksim peaced out of 'DWTS' back in February. Since then he's been running several dance studios, and starring in "Forever Tango" on Broadway.