Daniel Koh, who's chief of staff to Boston Mayor Marty Walsh, will leave the administration, and David Sweeney, currently the city's chief financial officer, will take Koh's place, the mayor's office said in a statement Friday.

Koh — who's reportedly expected to run for Congress in the newly vacant 3rd district — will depart Boston City Hall on Sept. 1. The 32-year-old is a native of Andover, which is in the 3rd Congressional District.

In the statement, Walsh lauded Koh's "unwavering service to the City of Boston for almost four years."

"Serving as Chief of Staff has been an incredible honor and I am grateful to the people of Boston and Mayor Walsh for the opportunity," Koh said in the statement.

Walsh said Sweeney, who lives in Dorchester, has been "an integral part of my administration from day one."

In the statement, Sweeney said: "I look forward to bringing the same focus of efficiency and accountability that has been applied to the Mayor's management of city finances to the role of Chief of Staff, and implementing the priorities that will benefit Boston and its neighborhoods."

Before joining the Walsh administration, Sweeney worked as chief financial officer for the state Lottery and prior to that was state budget director for the House Ways and Means Committee.

First-term Mayor Walsh faces re-election this fall. His primary challenger is Boston City Councilor Tito Jackson.

U.S. Rep. Niki Tsongas, who represents the 3rd district, announced last week she won't seek re-election. As WBUR has reported, a number of potential candidates have already expressed interest in the open seat.

Tufts University political science professor Jeffrey Berry expects Koh to run.

"Koh signaled very directly, if on background to the media, that he's resigning to run for the Democratic nomination in the 3rd district," Berry told WBUR, "and he's a viable candidate and maybe the early favorite getting out of the gate."

With reporting by WBUR's Newscast Unit