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.

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