Originally a shipyard electrician in the northern port city of Gdańsk, Wałęsa became a symbol of the historic changes that ended the Cold War, leading the Solidarity trade union movement which brought about the switch to a free-market economy in 1989.
"I can't believe it ... I am infected," the 78-year-old Nobel Peace Prize laureate wrote on Facebook.
Wałęsa said he had received three doses of COVID-19 vaccine, but that despite this he was feeling very ill, suffering from a headache and unable to warm up his body.
"I feel my flesh tearing away from the bone... I can't feel my own body," he wrote.
Wałęsa's spokesman said he was at home. He said the former president had been at his office earlier in the day but took a COVID-19 test after feeling unwell.
Wałęsa has suffered from ill health in recent years and underwent a heart operation in 2021.
He served as president from 1990 to 1995, the first leader of post-Communist Poland.
In recent years he was a staunch critic of Poland's ruling nationalists Law and Justice (PiS), who in turn have been deeply critical of the transition from Communism to a free-market economy that Wałęsa led.
(Reporting by Alan Charlish, Anna Koper, Pawel Florkiewicz; Editing by Philippa Fletcher, Alison Williams and Jonathan Oatis)