Katherine Marlea Clark (born July 17, 1963) is an American lawyer and politician who has served as the U.S. representative for Massachusetts's 5th congressional district since 2013. She has been House Minority Whip since 2023 and was Assistant House Democratic Leader (known as the "Assistant Speaker") from 2021 to 2023. Her district includes many of Boston's northern and western satellite cities and suburbs, such as Medford, Framingham, Woburn, Waltham, and her home city of Revere. Clark was a member of the Massachusetts House of Representatives from 2008 to 2011 and the Massachusetts Senate from 2011 to 2013.

Born in Connecticut, Clark worked as an attorney in several states before moving to Massachusetts in 1995, where she worked in state government. She joined the Melrose School Committee in 2002, becoming committee chair in 2005. She was first elected to the state legislature in 2008, and contributed to legislation regarding criminal justice, education, and municipal pensions. She is in her sixth term in Congress, having won the 2013 special election for the U.S. House of Representatives to succeed Ed Markey in the 5th district, and sits on the House Appropriations Committee.