Plaid Cymru has suspended the Carmarthen East and Dinefwr MP Jonathan Edwards from its Westminster group.
The party says the move follows Mr Edwards' arrest when police were called to an incident on May 20.
He was later released without charge pending further inquiries.
His party says withdrawing the whip is a "neutral act" during the police investigation.
A spokesperson said:
Dyfed Powys Police said a 44 year old man from Ammanford was arrested on Wednesday. He has been released on bail and the investigation is ongoing.