Officers were called to Stanley Park at around 5.40pm on Saturday, August 5 following the incident.
Police say that the girl was playing near the memorial gardens at around 4.40pm when she was approached by a man.
The man is then reported to have touched her inappropriately before the victim pushed him away. The man then made off from the scene.
The suspect is described as white, 5ft 7in tall, of medium build aged in his late 40s. He had short ginger hair, bushy ginger eyebrows and a moustache. He was wearing a t-shirt with yellow and purple stripes as well as dark blue jeans and black trainers.
Detectives are now appealing for information and urging anyone who recognises the man to contact police immediately.
Det Con Angela Scanlon, of Blackpool CID, said: “We are investigating after a 12-year-old girl reported being touched inappropriately in Stanley Park, Blackpool.
“The incident has left the victim upset and distressed and it is important we find this man as soon as possible.
“The girl has provided a very detailed description of the offender. I would ask anyone who recognises the man described, or who saw such an individual in the park around the time of the offence, to contact police.”
Anyone with information can call police on 101 quoting crime reference WA1714190.
Alternatively independent charity Crimestoppers can be contacted anonymously on 0800 555 111 or online at Crimestoppers-uk.org.