A knife-wielding masked man who sometimes stakes out his victims has robbed 21 businesses in a nearly yearlong crime spree, including two just last weekend, Nassau County police said late Tuesday.

The man, armed with a large kitchen knife and wearing either a hooded black or white sweatshirt, and a ski mask or scarf, is wanted in robberies in seven communities, including gas stations and convenience stores in Valley Stream, Franklin Square, Elmont and Lynbrook between Feb. 9, 2015, and Sunday.

“This subject is brazen and aggressive,” Nassau Det. Lt. Richard LeBrun, a police spokesman told reporters late Tuesday. He said the suspect had tussled with a Valley Stream convenience store owner, slashing him in the arm in February.

“We believe that he’s staking out the store to make sure that there’s nobody else in the store most of the time,” LeBrun said. “And he’s very quick.”

Police are seeking the public’s help in apprehending the suspect, who authorities believe lives in the local area. LeBrun said catching the suspect is one of the department’s “highest priorities.”

The suspect usually shows his knife and demands cash. He has taken entire cash drawers with him and also has taken cigarettes, favoring the Newport brand.

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After the robberies, he dashes off on foot in most cases and once on a bicycle, police said. Witnesses have also said they’ve see a white car, possibly a Honda Accord or Hyundai, yellow New York license plates in the area, LeBrun said.

In one instance, he robbed one Valley Stream store four times over two months, a tally from Nassau police shows.

Police have flooded the area in an effort to apprehend him, including using plain clothes officers and members of a criminal intelligence rapid response team. Authorities also released images of the suspect taken from video surveillance at more than one of the businesses.

He is described as thin, between 5-foot-10 and 6 feet tall and known to wear gloves and white-soled sneakers, police said.

He last struck on 2015 on Christmas Eve, then waited more than 2 weeks to surface again.

In his latest crime, police said the suspect robbed a Valley Stream convenience store and gas station over the weekend.

On Saturday, he first hit a 7-Eleven at 190 N. Central Ave., at 3:40 a.m., police said; at 8:20 p.m. Sunday, the suspect robbed a Valero gas station at 192 E. Sunrise Hwy.

John Maywald, a manager of the 7-Eleven, said the suspect snuck up behind an employee standing behind the register and demanded money. The man’s face was covered with a hood and mask.

“All you could see was his eyes,” Maywald said.

Maywald, who was not in the store during the robbery, said the suspect made off with $250 to $300, lottery tickets and about 15 to 20 boxes of Newport cigarettes.

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“I just want to get him off the streets. . . . He’s a menace to many people around Valley Stream and Lynbrook,” Maywald said.

One of his employees, who asked not to be named, said there are usually two people working, but he worries about his safety.

“I’m scared,” said the man, who was not working during the robbery.” You’re always watching who’s coming in and out.”

But Sandy Singh, 24, an employee at the Valero who was not working during the robbery on Sunday, said he is not shaken.

“We have two guys here every day,” he said, in between selling cigarettes, lottery tickets and gas to a slow but steady stream of customers on a cold night.

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In July, Newsday reported that police were looking for the same man who investigators believed at the time was responsible for nine such robberies starting with a robbery of a Shell gas station in North Woodmere on Feb. 9, shortly before 9:30 p.m.

In that robbery, the suspect made off with $400, said employee, who said that he managed to slip out the store and borrowed a phone to call police.

Police said 13 of the robberies took place in Valley Stream.

Among the robberies that took place early-to-mid-last year, the suspect struck a Valley Stream pharmacy on April 7 and a Franklin Square Carvel on July 11.

He robbed a Valley Stream beauty store at 12:40 p.m on July 14. That day, he robbed Charlene’s Stationary in Valley Stream at 3:56 p.m. He robbed the store again on July 20, July 31 and Sept. 12, police said.

— With Gary Dymski and Jo Napolitano