Lopez (D-Brooklyn), beset by allegations that he routinely groped and harassed young female staffers, sent a one-sentence letter to Assembly Speaker Sheldon Silver (D-Manhattan), saying he will step down.
"I hereby resign the public office of Member of Assembly from the 53rd Assembly District, Kings County, effective 9 a.m. Monday, May 20, 2013," he wrote.
Silver -- whose handling of the matter has been widely criticized -- had planned to ask the full Assembly Monday to consider a resolution to expel Lopez. It would have been the first time the Assembly expelled a member since 1920, when five Socialists were kicked out amid the "Red Scare."
Assemb. Amy Paulin (D-Scarsdale) said Saturday that there was "absolutely no doubt" Lopez would have been expelled.
Lopez, 71, while insisting on his innocence, had said Friday that he would serve through the end of the legislative session in June, and then resign to run for New York City Council. But that only outraged lawmakers further. With momentum building for his immediate ouster, Lopez chose a quicker exit.
It wasn't immediately clear whether he would still seek a seat on the City Council.
Despite the resignation letter, the Legislative Ethics Commission will continue to consider whether Lopez's conduct violated the state Public Officers Law, commission co-chairman Assemb. Charles Lavine (D-Glen Cove) said Saturday. An ethics law enacted in 2011 provides a window for sanctioning lawmakers even after they leave office.
A Silver spokesman declined to comment. A spokeswoman for Gov. Andrew M. Cuomo called it the "best" outcome.
"Today's immediate resignation is the best end to this ugly chapter," said Melissa DeRosa, Cuomo's communications director. "Now, we must do everything we can to ensure this type of behavior is never tolerated or allowed to occur again."
Lopez, in his 29th year in state office, has been under fire since last summer, when an Assembly ethics panel censured him after investigating complaints from two of his former staffers. Silver subsequently acknowledged he had approved a confidential agreement for $135,000 ($103,000 in public money, the rest from Lopez) to settle previous claims by two other staffers.
That disclosure prompted separate civil and criminal investigations. The findings last week were that the assemblyman forced physical contact with female staffers, pressured them to give him hand massages, send him flirty messages and stay in the same hotel room as his on overnight trips.