The incident happened on Tuesday at about 13:15 in Annan at the junction of High Street and Bank Street.
A man walked from the pavement and kicked the motorcycle, knocking it and the rider to the ground.
The attacker then kicked and punched his 26-year-old victim before walking off up Downies Wynd. The biker was left "bruised and shocked" by the assault.
PC Sarah Smith said: "This was a horrific attack in broad daylight in the centre of Annan at a busy lunchtime.
"We are following a positive line of inquiry however we are appealing for anyone who saw this attack to contact us at Annan to help us with this investigation.
"The victim received bruising and grazing as a result of being assaulted and was also left in a shocked and distressed state."
A motorcyclist has been attacked in broad daylight while stopped at traffic lights in a south of Scotland town.