Allison Coss and Scott Sippola allegedly told Stamos, 46, they had photos of him with cocaine and strippers and would sell them to the tabloids. Federal prosecutors said they never had the compromising pictures.
The couple were convicted of conspiracy and using e-mail to threaten a person's reputation. The charge carries a sentence of up to five years in prison.
At trial, a lawyer for Coss, 24, also claimed she had a romantic encounter with the ER actor when she was 17 but Stamos's attorney said the allegations were "simply untrue," the Associated Press reports.