The van driver who opened his door in Brooklyn, leading a cyclist to swerve and be fatally hit by a commercial truck Monday, said he feels “very sad” about the incident.

The driver, who refused to give his name, said at the scene of the crash in Sunset Park that he had just finished making an auto-glass delivery when he got inside his gray van on Third Avenue near 35th Street “and remembered I had to go out again.

“I was trying to get out again,” the driver said. “The cyclist hit [the van door], and he or she lost control after I opened it. I closed it and opened it again.

“It was just a matter of just closing the door and opening it again,” the van driver said. “I feel very sad.”

The motorist admitted that he did not look to check the roadway before he opened the door.

“I’m supposed to look 100 percent to see if someone is coming,” he said. “But if there was at least a bike lane, it would be much safer.”

The bike victim, Em Samolewicz, 30, was hit by a truck while riding north on Third Avenue just after 9 a.m. when she swerved into the road to try to avoid the opening car door, police said.

Medics rushed her to NYU Langone Hospital–Brooklyn, where she died, cops said.

Witness Gustavo Amarante, 55, of Washington Heights said he “heard like a boom” when the crashed happened and ran over.

“I ran back inside. I never see something like that before. Poor thing,” he said.

The truck driver, who was in a vehicle marked Gold Star Carriers — an Illinois-based company — stayed at the scene, and no arrests have been made, according to police.

The fatal incident marked the 18th cyclist death this year.

Assemblyman Felix Ortiz (D-Brooklyn) called it “a very sad day for the cyclist community,” “a very sad day for the people of Sunset Park, and “a very sad day for the victim’s family.”

Ortiz called on the de Blasio administration to do an “assessment” of 3rd Avenue in order to prevent tragedies like Monday’s.

“I hope that this tragedy can be used in a positive way to ensure that this administration will be able to do an assessment and analyze what kind of cyclist lane can be built on 3rd Avenue,” Ortiz said.

“Do what is right and make sure this is part of the master project you [Mayor de Blasio] have for Vision Zero,” the assemblyman said.