Mayor Bill de Blasio chided the NYPD on Monday for trying to use a post-9/11 anti-terrorism law to determine who leaked crime scene photos to a New York Post reporter.

“Yeah, I think it was a mistake,” de Blasio said on NY1’s “Inside City Hall,” referring to the department’s decision to subpoena Twitter on Dec. 9 for electronic information associated with the account of Post Police Bureau Chief Tina Moore.

“I think the effort to ensure that information that is not public is kept confidential — that is fair. But subpoenaing a reporter in that fashion? I am not comfortable with that. Freedom of the press really matters.”

The NYPD withdrew the subpoena last Wednesday — after Twitter alerted The Post, and the newspaper’s lawyers promptly contacted the department.

“I think the NYPD should not have done it, and I am glad they dropped that,” added de Blasio, who has a long history of verbally sparring with reporters.

The subpoena came amid an NYPD push to stop the flow of sourced information to the press.