At around 4pm on March 2 the man was approached by a man and woman in St Ann’s Well Garden, police say.
According to Sussex Police, he was forced to hand over his belongings before being assaulted - he suffered minor injuries.
Officers are appealing to anyone who witnessed anything suspicious in the area at the time.
Inspector Dee Wells said, “This incident happened at a busy time of day when there was likely other people in the park. We are sure someone may have seen something that could assist with our enquires.
“Violence will absolutely not be tolerated and we seek to establish the circumstances of this incident and trace those responsible.”
Anyone with information can contact Sussex Police online or call 101, quoting 888 of 02/03.