The 76-year-old was walking in St George’s Road, at 1.15pm, when she was attacked from behind.
She sustained severe bruising to her leg, and was taken by ambulance to Worthing Hospital.
The suspect is described as a white, aged between 30 and 40, 5ft 9 and of stocky build.
He was wearing a black top, dark trousers and dark sunglasses.
He ran off along Church Walk towards Farncombe Road after the attack.
Police want to speak to anyone who may have seen the man following the victim along Brighton Road from the town centre prior to the attack, or making-off along Church Walk afterwards.
Witnesses should call 101 quoting serial 0703 of 19/11.
Alternatively, call the independent charity Crimestoppers anonymously on 0800 555111.