Kal Penn -- aka Kumar from that "White Castle" movie -- was robbed at gunpoint  early this morning while walking in a neighborhood in Washington D.C., TMZ has learned.

Law enforcement sources tell TMZ Penn claims a man carrying a gun approached him around 1:20 AM and took his wallet and other personal property.

We're told Penn immediately reported the incident to authorities, who have launched an investigation into the matter.

Multiple calls to Penn's rep were not returned.

Go to TMZ for more.