It's not always sunny when you're Rob McElhenney ... which is exactly why the veteran actor is changing his name legally.
The star filed documents in L.A. to formally update his moniker ... cutting down on the number of syllables in his name by going by "Rob Mac."
Like we said, this isn't just a stage name change ... we're talking full-on new name in the eyes of the law -- a pretty substantial move for a dude of Rob's stature.