The 15-year-old victim was cycling in East Street, by The Turks pub, at 3.40pm on 5/2 when he was approached by a man who grabbed hold of the handlebars of the bike and thrust a knife towards him.
This caused the victim to jump backwards off his bike, which the man then took and rode away, turning right on to the London Road towards the Tesco Express in Crown Street.
The boy was not injured.
The offender is described as a white man in his mid to late twenties, who was 5ft 7ins tall and had a thin face. He was wearing a dark material jacket with hood.
The stolen bicycle was a grey and orange VooDoo Bantu mountain bike, frame number 447022479.
Investigating officer, Detective Constable Andrew Carroll, of Reading CID, said: “Although the boy was not injured, he was left shocked and distressed by this incident and we are keen to catch the man responsible.
“The incident happened shortly after the end of the school day and there would have been a lot of potential witnesses in a busy route in the centre of Reading.
“If you saw the offender cycling in the area, or have information that can help our investigation, please call the Thames Valley Police non-emergency number, 101, quoting reference 43180037985.
“Alternatively, you can call Crimestoppers anonymously on 0800 555111.”