Full House and ER star John Stamos ensnared in a blackmail plot over compromising photos snapped at a wild  2004 party.

Federal agents arrested two Michigan residents on December 2 after they allegedly threatened to release the snaps of Stamos unless he forked over $680,000 to the alleged extortionists.

Instead the hunky thesp went to authorities and helped snare alleged blackmailers Allison Coss, 23, and Scott Sippola, 30, in a sting operation, according to a FBI affidavit obtained by The Smoking Gun.

According to the legal docs, Stamos attended a Florida party with Coss and her pal when the allegedly compromising pix were snapped.  The exact nature of the images in question are not known.

Stamos later received an email in November from an unknown source threatening to “harm his reputation” by selling the shots to media. The star’s lawyer then contacted the FBI who set up the sting op.

A fed pretending to be Stamos’ rep arranged a money exchange with the anonymous e-mailer at the Princeton, Michigan airport.

When Coss and Sippola appeared they were promptly  arrested by feds and were released on bond on December 3.