The undercover officer who fatally shot a Brooklyn gun dealer this week is a hero detective and Liberty Award Medal finalist in the Finest category.

The 33-year-old officer, called as Detective X to guard his ID, was nominated last year for getting 153 illegal guns off Brooklyn streets.

Wednesday night, he went to Bushwick carrying $4,200 to buy six handguns from Alex Mercado, 22, an arms trafficker, police said. When the seller’s partner, Lucas Muniz, 31, pointed a .380 semiautomatic at the younger dealer and then at the undercover cop, the officer fired three times, fatally hitting the 31-year-old man twice in the chest, police said.

Last year, the detective told The Post he was gratified knowing he’d gotten so many guns off the street.