According to Sussex Police, the victim was approached by a man 'who grabbed her with both hands' and 'attempted to bring her to the ground' on Neville Road.
The incident reportedly happened around 10pm on Wednesday, May 4.
Police said the man ran off after the woman screamed and, as a result of the attack, the victim sustained bruising.
The suspect is described as white, 5ft 10in tall, slim, and was wearing a black beanie style hat, police said.
The lead Investigator for the case, Melanie Edwards from Coastal CID, said: “This would have been distressing for the victim, everyone should feel safe while walking the streets.
"I would be keen to reassure people this is an isolated incident in Bognor.”
Anyone who saw a man run off in the vicinity of the assault or heard the woman’s scream is asked to report it to police online or call 101 quoting serial number 1060 of 10/05.
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