A Rikers Island inmate about to be transferred to a clinic bit off part of a correction officer’s thumb, officials said yesterday.

Joseph Zilsaint, 30, was in a holding cell when the 46-year-old unidentified guard approached him at 10:45 p.m. Thursday.

Doctors at Bellevue Hospital were unable to reattach the thumb, but did treat two other guards injured by the same inmate.