Genevieve Davis, a financial consultant from Norwell, will be the Democratic challenger to longtime Republican State Senator Bob Hedlund in the November election.
Davis won the primary nod Thursday night over Hull social worker Steve May.
Davis will be on the ballot against by nine-term incumbent Republican Robert Hedlund of Weymouth.
The Plymouth-Norfolk District includes Weymouth, Hull, Hingham, Cohasset, Marshfield, Duxbury, Norwell and Scituate.
The general election is Nov. 6.