"Today, I tested positive for COVID-19," the former presidential candidate wrote in a series of tweets. "While I'm currently experiencing mild symptoms, I'm grateful to have the protection of a safe and effective vaccine -- and I know without it, this illness could be much, much worse."
"What we have learned over the last year and a half is that we are in this together, and I urge all Ohioans to help us crush this pandemic by wearing a mask and getting vaccinated so that we can get back to normal," he added.
The lawmaker is quarantining at his home in Howland, Ohio, and is "following all safety and medical precautions advised by his doctor," Ryan's office said in a news release.
He tested negative for COVID-19 three times in the previous 10 days before Monday's positive results, they said.
Ryan is the eighth member of the House to test positive for COVID-19 since early August, CNN reported. Dozens more have recorded positive tests since the start of the pandemic in March 2020.
He sought the 2020 Democratic presidential nomination among a large slate of contenders, including now-President Joe Biden, before dropping out in October 2019. He positioned himself as a "progressive who knows how to talk to working class people" who have drifted away from the Democratic Party.