John Chapman "Chap" Petersen (born March 27, 1968) is an American politician. A Democrat, he served in the Virginia House of Delegates 2002–06, was elected to the Virginia State Senate in November 2007, and was reelected in 2011. He represented the state's 34th district, made up of the city of Fairfax and large parts of Fairfax County. In June 2023, Petersen lost the Democratic primary for District 37 to Saddam Azlan Salim. Salim won the general election, and assumed office on January 10, 2024.

Petersen was regarded in office as a political moderate, and broke with his party on several issues.

Petersen graduated from Fairfax High School in 1986. He received a B.A. degree from Williams College in 1990 and a J.D. from the University of Virginia School of Law in 1994, when he began practicing law. He is currently^[update] a partner with the law firm of Chap Petersen & Associates, PLC.

Petersen's wife Sharon Kim (born September 10, 1970, Daegu, South Korea), is a practicing attorney; the couple have four children.

Petersen served on the Fairfax city council 1998–2001. He was elected to two terms in the House of Delegates, both times (2001 and 2003) defeating his predecessor, Republican Jack Rust.

In 2005, Petersen ran for Lieutenant Governor of Virginia. He was defeated in the Democratic primary, finishing third with 22% of the vote in a four-way race, behind State Senator Leslie L. Byrne and State Delegate Viola Baskerville, but ahead of State Senator Phil Puckett.

In 2006, Petersen was a senior advisor to Democratic Senate candidate Jim Webb, who defeated incumbent Republican Senator George Allen.

Petersen announced his candidacy for the 34th district seat in the State Senate, Wednesday, January 3, 2007. He defeated incumbent Republican Jeannemarie Devolites-Davis, wife of Congressman Thomas M. Davis, in the November 2007 election, taking 55% of the vote. The district had been the most Democratic state senate district held by a Republican. ^{[citation needed]}