The boy, who was part of a group, when he encountered a group of girls in West Belfast. The incident happened at around 7.45pm on Tuesday.
The girl was hit by a Ford Fiesta in the Stewartstown Road area, close to a roundabout.
PSNI Sergeant Hakin said: “It was reported that one of the males ran towards one of the females with a lit firework.
“As she attempted to avoid the male, she collided with the driver’s side of a grey Ford Fiesta which was driving close to the McKinstry Road roundabout.
“The female was taken to hospital for her injuries.
“Enquiries are continuing and we would appeal to anyone who witnessed the incident, or anyone with dashcam footage taken from the area at the time to contact police on 101 quoting reference number 1742, October 19, 2021.”
Incidents like this have been a recurring theme in recent weeks and police have issued warnings about Bonfire Night approaching.
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Anyone over 18 can buy, carry or use fireworks, while those over 16 can use lesser fireworks – known as ‘throwdown’ – which include party poppers and firecrackers.
Police continue to carry out enquiries into the firework incident in West Belfast.