In an aptly described “joint policy initiative,” Mayor de Blasio and Police Commissioner Bill Bratton announced Monday that most people caught with small amounts of marijuana in the city will get slapped with summonses instead of arrested.

The historic move marked a public about-face for Bratton, who earlier this year vowed NYPD practices wouldn’t change after the Brooklyn district attorney’s decision not to prosecute first-time offenders for low-level pot possession.

Sources said the new policy was ordered by de Blasio, whose campaign promises included ending marijuana arrests that overwhelmingly targeted minorities.

“It began with the mayor, as his initiative,” said an elected official familiar with the issue. “It was something very dear to the mayor that had to happen, no matter what.”

Bratton, the sources noted, did not resist de Blasio’s directive.

The NYPD declined to comment. City Hall credited both men with the idea.

“This is a joint policy initiative, developed and implemented by Commissioner Bratton and the leadership team at the NYPD, and fully supported by the mayor,” a de Blasio spokesman said.

Joined by de Blasio at 1 Police Plaza, Bratton said he and other police officials had been working on the change in policing tactics for “the last number of months.” Under the new policy, people busted for possessing 25 grams or less of pot — a violation under New York law — will no longer be cuffed, hauled off to a police station, fingerprinted and booked.

Bratton held up a plastic sandwich bag stuffed almost full of oregano to illustrate how big a stash was at issue.

“To give you, the public, a sense of what 25 grams of marijuana would look like, that’s about it,” he said.

“It’s not a large amount.”

Chief Thomas Purtell of the NYPD’s Organized Crime Control Bureau put the value of that much actual weed at $300.

The new policy covers only marijuana that appears to be for “personal use,” rather than for sale. Anyone caught smoking in public will still be subject to arrest regardless of the amount of pot in view, the commissioner said.

Other exceptions to the no-arrest rule include people with open arrest warrants, those wanted in connection with active investigations or pot carriers who don’t have identification.

Bratton said the policy would be put into writing and a training video would be shown to cops during roll call before it goes into effect on Nov. 19.

But law-enforcement sources said an unwritten version of the policy was enacted “on the command level” about a year and a half ago under then-Police Commissioner Ray Kelly.

“If you saw someone smoking a joint, you would bring them back to the precinct and give them a summons if they didn’t have a record,” a source said.

Kelly declined to comment Monday.

The new policy effectively neutralizes Brooklyn DA Ken Thompson’s refusal to prosecute low-level pot arrests, which he said were clogging the justice system and taking “an unnecessary toll on offenders.”

It also maintains a revenue stream for the city, because the summonses carry fines of up to $100 for first-time offenders and up to $250 for second-timers.

In a statement, Thompson said, “I applaud Mayor de Blasio’s decision.” which he called “similar to the approach that I have taken in Brooklyn.”

But he said he was “concerned about the due-process rights” of Brooklynites who get issued pot summonses, because almost all Brooklyn violations are prosecuted in Manhattan Criminal Court.

De Blasio described the new policy as a win-win, especially for young minorities, saying, “We don’t want to saddle people who have made one small mistake with something that will follow them for the rest of their lives.”

“It will certainly be good for New Yorkers of color, and particularly young people of color,” the mayor said.

“When an individual is arrested, even for the smallest possession of marijuana, it hurts their chances to get a good job, it hurts their chances to get housing, it hurts their chances to qualify for a student loan — it can literally follow them the rest of their lives and saddle young people with challenges that for many are very, very difficult to overcome,” the mayor said.

“I think you will see fewer unnecessary arrests,” he said.

Monday’s announcement followed a Post report last week that revealed the NYPD had stopped marijuana “buy and bust” operations to focus on harder drugs.

Bratton called the article “false” Monday while admitting he had changed strategy to target only “significant” drug sales.

Law-enforcement sources on Monday confirmed the accuracy of the Post report and said they weren’t surprised by Bratton’s attempt to dance around the issue.

“You can’t go on the record that you’re pulling back on marijuana arrests when it’s a law on the books,” one source said.

Additional reporting by Jamie Schram, Shawn Cohen and Ben Feuerherd