Cops have arrested an accomplice in the Veterans Day heist in the Diamond District, police sources said.

Rondu Frisby was picked up at his apartment on West 147th Street in Harlem on Monday, the sources said. He regularly visits the location of the heist, Watch Standard Jewelry on West 47th Street, and sometimes did odd jobs there, the sources said.

He was charged with robbery, criminal possession of a controlled substance and criminal possession of marijuana, cops said.

Cops were grilling Frisby because he buzzed in a gunman who then robbed the jewelry shop after pretending to be a deliveryman, the sources told The Post.

Shop owner Daniel Mikhaylov said Frisby was there for only five minutes before the robbery, and called him a friend, but nothing more.

Police were still searching for the alleged gunman, identified Monday night as Leon Fenner, 58.

He was seen on the video sauntering around outside the building with his head down, looking at his phone and apparently trying to blend in with the crowd.

“Rondu’s not my employee, I want to make that clear,” Mikhaylov said.

“He is a person that comes by once in a while, like a friend. He started out as a customer who’d buy gifts for his girlfriend.”

Mikhaylov, 33, and other employees were ordered to the floor by the gunman, who pistol-whipped Mikhaylov’s father, Boris, during the robbery.

“[There was a] gun pointed at my sister’s face. My father got hit in the head,” he said.

“It’s family. My dad’s still walking around with a big bump on his head.”

Cops flooded West 47th Street after the mid-afternoon heist, which occurred as tourists packed Fifth Avenue down the street for the Veterans Day Parade.

Surveillance video shows the robbers before they entered the eighth-floor business, and then quickly walking away toward Sixth Avenue afterward.

The gunman was wearing a trench coat, Kangol hat and a goatee, and was carrying a duffel bag.

A rival jeweler admitted to cops that he sent the two goons who held up the shop, one acting as a lookout.

But the rival said the thugs were only supposed to scare Mikhaylov instead of getting greedy and robbing the joint.

“They wanted to show they had muscle,” a law enforcement source said of the rival and his associates.