The West Virginia Democrat, who faces a tough re-election this year in a state Trump won by about 40 percentage points, became the first member of his party to commit to supporting Haspel. It improves her chances of leading the U.S. intelligence agency.
"I have found Gina Haspel to be a person of great character," Manchin said in a statement, saying he looks forward to "working with [her] to ensure the people of West Virginia and the United States are kept safe."
Democratic Sen. Jon Tester of Montana, another senator facing a difficult re-election bid this year, previously said he will not back Haspel.
Manchin questioned Haspel on Wednesday as part of her confirmation hearing before the Senate Intelligence Committee. Earlier, he said she did a "great job" in the hearing.
Haspel has faced criticism from opponents, including many Democrats, for overseeing a CIA detention facility in Thailand at which a suspect was waterboarded. She has also faced backlash for the destruction of recordings of interrogations.
On Wednesday, she tried to assuage lawmakers' concerns about the potential for her to allow the use of so-called enhanced interrogation techniques, which were used after the Sept. 11, 2001, terror attacks but have since been cracked down on in the U.S. As a candidate, Trump said he supports the use of waterboarding or worse tactics that are broadly considered torture.
Repeatedly pressed about whether she would allow tactics she considers immoral, Haspel at times evaded senators' questions and noted that the CIA does not have a formal role in interrogations. She said she would not restore "under any circumstances" an interrogation program at the CIA and supports the "higher moral standard" the U.S. holds now.