'Jimmy Neutron' Star Jeff Garcia Hospitalized

"Jimmy Neutron" star Jeff Garcia is in the hospital ... and it doesn't sound good.

Family sources tell us Jeff is currently hospitalized in Southern California, but it's unclear what is ailing him.

We're told Jeff was previously hospitalized around November 20 with pneumonia ... but he recovered and was discharged a few days later.

Jeff is most famous for his voiceover work in the "Jimmy Neutron" franchise ... Jeff was the voice of Sheen in 2001's "Jimmy Neutron: Boy Genius" ... and he also voiced Sheen in 52 episodes of the TV show "The Adventures of Jimmy Neutron: Boy Genius."

His "Jimmy Neutron" character even got his own spinoff show, "Planet Sheen," in 2010, and Jeff did voice work in 26 episodes.

Jeff's also a standup comic ... and he had been doing comedy work as recently as November ... a couple weeks before he wound up in the hospital.

He's only 50 years old.