The FBI arrested two Michigan residents on 2 December (09) after they allegedly threatened to release the snaps of the ER star unless he handed over $680,000 (£425,000).
But Stamos went to authorities and helped them snare alleged extortionists Allison Coss, 23, and Scott Sippola, 30, in a sting, according to an affidavit obtained by The Smoking Gun.
According to the document, the actor attended a Florida party with Coss and her pal when the snaps in question were taken.
He received an email in November (09) from an unknown source threatening to "harm (Stamos') reputation" by releasing the shots to the media, at which point the star's lawyer contacted the FBI.
An agent posing as Stamos' representative arranged a money exchange with the anonymous emailer at the Princeton, Michigan airport, where Coss and Sippola were arrested.
They were released on bond on 3 December (09).
Stamos joins John Travolta, Cindy Crawford and David Letterman as the Hollywood stars who have been targeted by alleged extortionists in the past year (08-09).