Assemblyman Vito Lopez informed leaders on Saturday he will resign effective Monday morning.
Assembly Speaker Sheldon Silver had planned a rare expulsion proceeding against Lopez beginning Monday.
Lopez was once a powerful Brooklyn Democratic leader and has been in the Assembly's Democratic majority since 1984.
Two reports released last week detailed lurid accusations of sexual harassment of several women on his staff, but neither report found proof of a crime.
Silver was trying to expel Lopez on the basis of apparently violating Assembly policies against sexual harassment and retaliation.
The state ethics board has also accused Lopez civil violations of the Public Officer's Law.