Suffolk County Police Commissioner Rodney Harrison, who helped the department finally crack the infamous Gilgo Beach serial killings case, suddenly announced Thursday night that he is stepping down after nearly two years on the job, according to a memo obtained by The Post.

Harrison, a former NYPD chief of department, told officers he will inform Suffolk County Executive Steve Bellone on Friday about his decision to resign, the message states.

“Serving as the Suffolk County Police commissioner for the past two years was not only a privilege but one of the greatest highlights of my law enforcement career and that is because of you,” Harrison said in the memo.

“It has been an honor to be part of this organization and work with so many talented individuals,” he also said. “Each day, I have been impressed by stories of heroism, bravery, and dedication to the residents of this county. To say I appreciate and admire each one of you, is an understatement.”

A source told The Post the announcement Thursday was a surprise to the department. It’s unclear what his next move is or when his last day will be.

Harrison took the job in January 2022 and vowed his police force would work tirelessly to capture a suspect in the Gilgo Beach murders, which had gone unsolved for over a decade. He created a special joint task force dedicated to the cold case killings.

In July, Rex Heuermann was arrested near his Manhattan office and charged with the murders of three women in the case.

Heuermann, a Nassau County resident, has pleaded not guilty to the slayings of Melissa Barthlemey, Megan Waterman and Amber Lynn Costello.

He also is considered the prime suspect in the killing of at least one more victim, Maureen Brainard-Barnes.

Harrison took the commissioner job after leaving his post as chief of the department at the NYPD. He joined the Big Apple police force in 1991.

Harrison lauded the efforts of the Suffolk department Thursday and noted “I leave knowing Suffolk County is in good hands.”

Suffolk County police did not immediately provide a comment Thursday evening.