He described himself as a “bold” candidate, adding that it was “crucial” voters knew what the Greens “will always stand against”.
Polanski, who has accused Israel of “committing genocide” and “burning people alive”, said he wanted to lead “a party that will confront fascism and will call a genocide exactly what it is.”
Polanski previously claimed the violence in Israel and Gaza did not begin on October 7.
In his campaign video, he said he wanted to reach “millions more people” and claimed he would use his platform “to lift people up”.
“We know how to win now, but it’s time to aim higher,” he said, citing 39 constituencies where the Greens came second to Labour.
His unexpected leadership challenge follows the party’s strongest-ever general election performance, in which Denyer and Ramsay led the Greens to win four seats.
During last year’s general election campaign, after the JC had revealed extreme anti-Israel social media activity by Green candidates who were later dropped, Polanski denied that the party vetting problem and said any suggestion that it had a problem with antisemitic candidates would be a “huge overstretch”.
A Green Party local councillor who was elected to Leeds city council last May celebrated his win by shouting “Allahu akbar”.
A former Liberal Democrat who joined the Greens in 2017, Polanski has a background in community theatre and is sometimes used by the party for media appearances.
