“I’m running for president to offer a better choice for the people outside the failed two-party system,” Ms Stein, 73, wrote on X.
“Political insiders always smear outsiders like us and try to shame voters who want better choices. But without freedom of choice in elections, there is no democracy,” she added.
She joins one other prominent third-party-affiliated candidate, Robert F Kennedy Jr, in trying to take on the expected Republican nominee, ex-president Donald Trump, and Democratic nominee, President Joe Biden.
Her announcement comes approximately two months before the first general election primary is set to take place in January.