A 50-year-old man who suffers from health problems and learning difficulties was knocked to the floor by two young women and robbed of cash in Boultham Park in Lincoln.
Officers are appealing for information in relation to the incident, which happened between 8pm and 9pm on the evening of Wednesday, October 29.
The victim was walking through the park after visiting his friend when he was reportedly approached by two young women who began to talk to him.
One of the women then knocked the man to the floor and stole £200 from his pocket.
The woman who knocked the man down is described as white, aged about 18-19 years old, and with short ginger fluffy hair.
It is thought that a woman was walking past the park with a dog at the time of the incident and called out at the girl to stop what she was doing.
Investigating officers are keen to hear from this woman as she may have information that proves vital to the enquiry.
In addition, anyone who saw what happened or has information they believe could be relevant is also asked to call Lincolnshire Police on 101 quoting incident 183 of October 30.
Alternatively contact the independent charity Crimestoppers anonymously on 0800 555 111.