For the last 20 years, New York City mayors have always marched in the parade, but Democrats don't because the parade won't allow gay groups to participate.
So at this year's event, de Blasio will not join in.
"I simply disagree with the organizers of that parade in their exclusion of some individuals in this city," said de Blasio.
"And I don't want to be associated with people who don't want to be associated with Irish Catholics. So it's just as well he's staying out," said the Catholic League's Bill Donohue.
Backers of the parade say good riddance to the mayor, but gay groups had asked for more. They wanted him to ban police and firefighters from marching in uniform. But the mayor refused to go that far.
"I've said what I think. I respect the right of our city workers to march in their uniform period," the mayor said.
The decision is a disappointment to many gay leaders, who had hoped for big change with de Blasio.
"He didn't have to come out today and say look, they can't march. But he also didn't have to say they're going to march period," said Paul Schindler of the Gay City News. "I think that shows he doesn't want to inject himself into this issue and he's missing an opportunity to show leadership."
So the parade this year goes forward pretty much as always. Uniformed city workers march, but the mayor won't.
"I think there is a tyrannical tendency in this mayor and I'm sure his natural impulse is to give into the fascists who are asking him to banning the police from going there," said Donohue "There would be an insurrection and he couldn't win. He knows that. He's not exactly stupid."
The last major Democrat to march along 5th Avenue in the parade was Hillary Clinton back in 2000. Ironically, her campaign manager then, was Bill de Blasio. .
The new mayor's predecessor, Michael Bloomberg, always marched. De Blasio also did not march while public advocate.
The parade's organizers say gay people are welcome to march, but they say signs celebrating being gay would detract from the parade's focus on honoring Irish heritage.
The parade is a tradition that predates the city itself. Organizers predict more than 1 million people will attend on March 17.