Joshua Morris, seven, was last seen with his brothers in a park in Bourne Road, Cosham, between 5pm and 6pm tonight.
The youngster is slim with blonde hair, which is cut quite short, and was wearing a dark hoodie and tracksuit bottoms with red paint on them.
He had a black scooter with purple wheels with him, Hampshire police said.
A police spokesman said: 'If you have seen anyone matching his description or have any other information that might help us, please call 999.'
Police have not yet released a photo of the boy.