"After a lot of conversation, reflection and prayer, I've decided that a 2020 campaign for president is not for me," Patrick, 62, posted on his Facebook page.
Patrick's statement was expected. Several sources had confirmed Wednesday that he would not run.
Patrick has traveled across the country in support of Democratic candidates in the recent midterm elections, raising his national profile.
He says he has been overwhelmed with advice and encouragement.
"But knowing that the cruelty of our elections process would ultimately splash back on people whom Diane and I love, but who hadn't signed up for the journey, was more than I could ask," he said.
Diane is his wife.
Patrick served two terms as governor, from 2007 to 2015, was assistant attorney general for civil rights in the Clinton administration, and since leaving the governor's office has been a managing director for Bain Capital.
This year, some of Patrick's supporters and close advisers started the Reason to Believe political action committee, "a grassroots organization dedicated to advancing a positive, progressive vision for our nation in 2018 and 2020." Reason to Believe PAC had been holding meetups across the country, including in early presidential primary states.
Patrick said he will stay involved in the political process.
"America feels more ready than usual for big answers to our big challenges. That's an exciting moment that I hope we don't miss. I hope to help in whatever way I can. It just won't be as a candidate for president," he said.
While Patrick has dropped out of the 2020 race, dozens of Democrats are considering jumping in, including nearly a half-dozen senators and several House members.
Patrick previously expressed some concerns about breaking through if he sought the nomination, telling David Axelrod, a former adviser to Barack Obama, that he wasn't sure he could stand out in such a large field.
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