Ryan announced his run in a letter to colleagues Thursday. House Democrats will hold leadership elections Nov. 30.
In the letter, Ryan noted that over the past 18 years, Democrats have only been in the majority in the House for two terms, and "last week's election results set us back even further," he said.
"Last Tuesday's election will forever be remembered as a major turning point for the United States of America. Like many Americans, I was disheartened by the results, but I also realized that Democrats must not let this opportunity for change pass by without a fight," the statement went on to say.
Pelosi is well-known for her ability to count votes. She said Wednesday she has the backing of two-thirds of the caucus. Ryan has dismissed that claim.
House Democrats gained a handful of seats in last week's election, not nearly as many as Pelosi had predicted.