A man was pushed to the ground before by two robbers and had his phone, wallet and cash stolen from him as he walked home along Victoria Street in Clayton-le-Moors.
The incident happened just before 10.30pm on Wednesday, February 9 as the victim, a 30 year old man, approached the canal bridge.
He wasn't seriously injured but has been left shaken by the ordeal.
Detectives are keen to trace a man and woman who were walking down Victoria Street together just before 10.30pm and may have witnessed the attack.
The woman was wearing calf length boots and was carrying a bag over her right shoulder.
Detective Sergeant Nigel Watson said: "If anyone was in the area at the time and witnessed something suspicious then I would be keen to hear from them. I also want to speak to that man and woman who were walking down Victoria Street. They may have important information that can help bring the offenders to justice."
If anyone has any information then they are asked to call Lancashire police on 0845 1 25 35 45 or call Crimestoppers anonymously on 0800 555 111.