Warren's progressive platform offers "a different vision of America than what we have right now," Rapinoe told CNN's Don Lemon on "CNN Tonight" as she called for "really big, bold, brave progressive policies."
What's more important than beating Trump, Rapinoe argued, is the "need to show people that there's a better way forward" on issues such as health care, child care and opioid treatment.
"I just feel like I trust her," Rapinoe said.
Rapinoe said she would love to help Warren's team in any way she can, "so consider me Team Warren."
"I think we got this, Megan," Warren said.
"Well, we have a little saying on the National Team that we like to say, I don't know if you can cuss on here, but it's LFG. So, let's do this," Rapinoe said.
Warren laughed and said, "Let's do this. All right, Megan, you and me, LFG."
Rapinoe told Lemon that Trump "obviously" should be impeached, though she wasn't sure "if we'll get there." She praised Democrats for having "done a really amazing job of trying to stick to the story and really give the truth to the American people."
"It's very clear -- he uses the office of the presidency to personally enrich himself, his family and the people around him," she said of Trump. "He abuses power, he cuts people out of the communication, he doesn't give all the information -- so for me it's impeachment for sure."
Rapinoe, who led the Women's National Team to victory as a co-captain in the World Cup in July, also won the Ballon d'Or award last week, given annually to the world's best soccer players.