Steve Landesberg -- famous for his roles on "Barney Miller" and more recently "Forgetting Sarah Marshall" -- has died ... this according to the actor's agent who spoke with TMZ.


We're told Landesberg died of cancer early this morning.  He had been battling the disease for a while.

Landesberg played Dr. Rosenbaum -- the pediatrician -- in "Sarah Marshall."  But he was more famous for his role as Det. Sgt. Arthur Dietrich on the TV show "Barney Miller," which ran from 1975 - 82.

Story developing ...