Lawmaker William Wong resigned on Friday following his arrest earlier in the week on suspicion of drink-driving.
Hours before, Wong saw his administrative duties suspended by Chinese University where he is associate dean of engineering.
Wong said he is deeply sorry for the trouble he has caused to both Legco and society in general.
"To avoid affecting the operation of the Legislative Council, I have decided after careful consideration to resign from all my posts in the Legislative Council with immediate effect," he said in a statement.
He added that he will continue to serve the public in a different capacity.
Wong, who was an Election Committee legislator, said he hopes people will understand that he cannot comment further on the drink-driving allegation because the matter is under investigation by the police.
He was arrested on Monday after his vehicle allegedly hit two parked cars at the university.
Legco president Starry Lee confirmed the council's secretariat had received notification of the resignation in writing and that it took effect on Friday.
Lee said she respects Wong's decision to step down and that she thanked him for his service to the council and Hong Kong as a whole.
Edited by Thomas McAlinden
