McKeon, 75, was elected from his Republican southern California district in 1992 and easily won re-election thereafter. He rose to chairman of the House Armed Services Committee in 2011.
"Mr. McKeon will be making his announcement tomorrow [Thursday] morning that he'll be retiring," a source familiar with his plans told the Hill, a Washington political publication.
Politico, which first reported McKeon would not seek reelection, said his retirement has been expected and former state Sen. Tony Strickland and state Sen. Steve Knight plan to seek the seat in California's 25th congressional district.
The Washington Post said Rep. Mac Thornberry, R-Tex., is considered a likely front-runner to succeed McKeon as chairman of the armed services panel.