A firebug tossed a Molotov cocktail at the Masonic Hall in Manhattan’s Flatiron District, police said Thursday.

The suspect, wearing a gray sweatshirt and green track pants, approached the historic building on 24th Street near Sixth Avenue at about 11:45 p.m. on Tuesday and chucked the glass “incendiary device” at an entrance, cops said.

He then ran off and has not been caught, police said.

The fire went out soon after it ignited, cops said. Firefighters were called to the scene, but the flames were already extinguished by the time they arrived, authorities said.

The building, which spans an entire city block, has been open since 1873, according to its website.

The building is home to a number of Masonic Lodges and rooms can be rented in it for private events.