"Under our current leadership, Democrats have been reduced to our smallest congressional minority since 1929. This should indicate to all of us that keeping our leadership team completely unchanged will simply lead to more disappointment in future elections," Ryan said in a statement announcing his bid.
Earlier Thursday, Ryan told CNN Pelosi does not have enough support to remain atop the caucus.
"The definition of insanity," he said, is to do the "same thing over and over again and keep getting the same results."
Ryan added: "I think there's a lot of support for going in a different direction. I really do."
Earlier, a quiet lobbying push emerged to recruit the fifth-ranking Democratic leader, Joe Crowley of New York, to challenge Pelosi. He said later Thursday he would not challenge her.
The discussions come as Democrats are struggling to chart a path forward after Trump's stunning victory last week. Even Pelosi loyalists, like GK Butterfield of North Carolina, say she needs to present a more specific vision about where she wants to take the party.
"Well, we've lost 60 seats over the last few years," Butterfield, head of the Congressional Black Caucus, told CNN. "We cannot continue to lose seats here in the House."
Leadership elections were already delayed until after Thanksgiving. She initially resisted shifting the date but was forced to back down after House Democrats demanded more time.
As Pelosi walked into a Democratic caucus meeting Thursday, she projected an aura of confidence.
"No," she told CNN when asked if she was concerned about a leadership challenge.
At a news conference later in the day, Pelosi predicted she would be overwhelmingly re-elected as the party's leader, saying she has the backing of two-thirds of her caucus. And she strongly defended her record, pointing to how the Democrats got back into the House majority in the 2006 midterms during the Bush years and suggesting the same would be true in the Trump years.
"I am very proud to have the opportunity" to get the Democrats back to the majority, she said.
Yet Democrats are still frustrated at Pelosi, according to members and aides.
"Instead of listening to members and hearing their thoughts about what went wrong, and what we should do instead, she instead presented a new agenda," one Democratic source said. "That is not what members want ... That is going to really frustrate people."
The 43-year-old Ryan, who joined the House in 2003, is trying to tap into the angst among some younger members who believe it's time to take the reins from the 76-year-old Pelosi. After Ryan entered Congress, Pelosi recruited him along to serve in a group to elevate the profile of younger members.
Now he believes it's time for the party to sell itself to the Rust Belt in light of Trump's victory in states decimated by the downturn of the manufacturing sector, saying he would bring a "real connection to blue-collar people" if he became Democratic leader.