The robber, who was armed with a pen knife, threatened the cyclist before stealing her handbag.
The incident left the 49-year-old woman with minor injuries, requiring medical treatment at Queen Alexandra Hospital.
A spokesman for Hampshire Constabulary said: ‘We were called at 5.17am following reports of a robbery in Holbrook Road, Portsmouth.
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‘A woman on a bicycle was threatened with a pen knife and minor injuries.
‘Investigations are ongoing.’
No description of a suspect were given at this time.
Anyone with information is asked to call 101, quoting 44190448771.