A heartless firebug torched a 99-year-old granny’s front lawn Friday in the Bronx, police sources told The Post.

The near-centenarian Unionport woman looked outside her window on Blackrock Avenue around 12:30 am to see a strange man pouring gasoline onto her garden and fence, and then igniting it.

The area went up in flames that spread to the sidewalk and a car parked in the driveway, authorities said.

No one was injured in the fire. The motive is still under investigation.

The suspect fled with a gas can in a red Toyota traveling westbound on Blackrock Avenue, according to police sources. Police describe the alleged arsonist as 5-foot-6 tall and wearing a gray hooded sweatshirt.

Additional reporting by Stephanie Pagones