Police said the victim was scooting through Thatcham in Berkshire with another boy when they were approached by a man on a bike.
The offender elbowed the victim causing him to fall into bushes, before kicking him and stealing his mobile phone.
Police said the "nasty unprovoked attack" in Station Road had left the boy "scared to leave his home".
The offender is described as white, 5ft 8ins (1.72m), aged in his 20s or early 30s, well built, with pale skin, dark circles under his eyes, short brown curly hair and big bushy eyebrows.
He was wearing a black bandana with red stars on it that was covering his nose.
He also wore a black hooded Adidas top, dark jogging bottoms with white stripes down each side that were ripped at the knees and black Nike trainers with blue soles.
Det Con Claire Midwinter, of Thames Valley Police, said: "This may have appeared to be youngsters messing around, we urge anyone to call us if they have any information which would help us identify a suspect for this nasty unprovoked attack."
The stolen phone was recovered in Horne Road, the force said.
A 12-year-old boy has been knocked off his scooter and kicked in the ribs by a man during a robbery.