Arizona Republican Trent Franks has resigned amid an ethics investigation into claims he repeatedly asked female staff to be surrogate mothers.
The announcement came after a congressional panel said it was opening an inquiry into sexual harassment allegations against Mr Franks.
The lawmaker acknowledged discussing surrogacy with two female aides when he and his wife were facing infertility.
He is the third member of Congress to resign in three days.
The Associated Press reports one of Mr Franks' former aides accuses him of offering her $5m (£3.7m) to act as a surrogate mother, repeatedly pressing her to carry his child.
She told the news agency that another female staff member had also been approached by Mr Franks about surrogacy.
One of the aides reportedly said Mr Franks retaliated against her after she turned down his alleged surrogacy requests by ignoring her and withholding assignments.
A spokesman for the eight-term congressman - who has a net worth of $33m - would not comment on whether he had offered aides money to act as surrogates.
Mr Franks said on Thursday his resignation would take effect next month.
But on Friday he said he had decided to quit immediately after his wife was admitted to a Washington hospital "due to an ongoing ailment".
It was revealed last week that Mr Farenthold used $84,000 of taxpayer money to settle a sexual harassment lawsuit with his former communications director.
Resignations of two Democratic lawmakers have shaken Washington this week, reports BBC.