A quirky thief wearing a surgical mask and Rollerblades robbed a Brooklyn deli at knifepoint — but was nabbed by quick-thinking customers who locked him inside the joint, law-enforcement sources said.

Igor Dubovoy allegedly whipped out a BB gun as he skated inside Belarus Grocery at 495 Neptune Ave. in Brighton Beach at 6:15 p.m. Sunday, the sources said.

Dubovoy, 21, allegedly demanded money from a female worker, who promptly opened the register, sources said.

He then allegedly scooped up five $20 bills and tried to skate away to freedom, but several customers shut the front door, locking him inside.

Police were quickly called and arrested Dubovoy, who was charged with robbery, menacing and possessing air pistols.

No one was injured.