Surveillance video shows an angry motorist “punching the s–t” out of an off-duty cop through a car window before the officer fatally shot him, the owner of a nearby building said Wednesday.

The owner of the Atlantic Avenue building in Brooklyn said he plans to give his video to the NYPD.

It shows 37-year-old Delrawn Small get out of his car at the traffic light at the intersection of Atlantic Avenue and Bradford Street and charge Officer Wayne Isaacs’ car — two cars ahead — the landlord said.

Small’s girlfriend Zaquanna Albert, 35, can be seen trying to hold her man back, but he could not be restrained as their 5-month-old son and one of Albert’s teenage daughters sat in the car.

Small pummeled Isaacs, 37, through his open driver’s-side window with “big haymaker” punches before the flash of gunfire can be seen in the video, said the landlord.

“The video shows the guy coming out of his f–king car, running up to [the cop’s] car, going in the driver’s-side window and just punching the s–t of this poor cop. Then all of a sudden you see sparks,” he said, adding that the next image is of Small hitting the pavement.

He added that Small “was wailing on [Isaacs’] face, like ‘pow, pow!’ He was looking to knock this guy out, punching and punching. Maybe four punches.

“It was big haymakers.”

Small’s girlfriend initially told police investigators that Isaacs exited his car and opened fire on Small, but police sources rejected that narrative.

Isaacs, who was heading home after a 4 p.m.-to-midnight shift in the 79th Precinct in Bedford-Stuyvesant, pulled out his service weapon from underneath his white T-shirt and blasted Small once in the head and once in the chest after the pummeling, sources said.

Small, an ex-con who has a lengthy rap sheet with 19 prior arrests, died at the scene.

Police sources told The Post that investigators have already recovered surveillance footage from a different, less visible vantage point that shows Small get out of his car and lean into Isaacs’ driver’s-side window before Small can be seen collapsing to the ground.

Family members of the slain dad told The Post on Tuesday that if Isaacs, a three-year veteran, is not charged for the killing of Small, they’ll “hunt him down” themselves.

Sources said cops will be posted outside Isaacs’ home for security.

At a vigil for Small on Atlantic Avenue on Wednesday night, the slain man’s brother said it didn’t matter if Small threw the first punch.

“At the end of the day, all that matters to me is that he got shot by a cop for no reason,” Victor Dempsey said.

The NYPD is investigating the fatal shooting along with the state Attorney General’s Office.

Additional reporting by Larry Celona and Yijun Wang