Suspected Gilgo Beach serial killer Rex Heuermann was caught on video strolling down a Midtown sidewalk in the moments before plainclothes cops swooped in to arrest him, footage shows.

Heuermann, 59, appeared oblivious as he walked along, with a messenger bag slung over his shoulder.

Suddenly plainclothes officers can be seen stepping in front of, then surrounding the architect and taking him into custody, according to footage obtained by WABC. 

The takedown occurred close to Heuermann’s Fifth Avenue office Thursday night.

Heuermann appeared in court Friday afternoon and pleaded not guilty to three counts of first-degree murder and other charges related to the deaths of three women over 10 years ago. The life-long suburbanite, who was emotionless in court, was also named the prime suspect in a fourth killing.

He was taken into custody after DNA found on the hair of victim Megan Waterman matched that of his, taken by investigators from a discarded pizza crust in January.

Investigators swarmed Heuermann’s ramshackle Massapequa Park home Saturday, scouring evidence in the rundown backyard and removing seemingly random items — cat food and a cat scratch pole; a framed picture; and empty book case.

Drone footage showed authorities sifting through the tiny backyard of the house for additional clues, with law enforcement clad in powder blue caps, white jumpsuit and blue rubber gloves — dotted the property in the 58-second clip obtained by the Daily Mail.

A young Long Island woman revealed Friday that she had a disturbing encounter with Heuermann earlier this month in a park — and that she was so shaken, she filed a police report.

It took police 18 months to build their case against Heuermann, whom they linked to the notorious killings through burner cell phone calls, sick Google searches, and DNA evidence.