The 14 year-old was cycling along St Michael's Road in Kirkham on Saturday September 2 at around 6pm when the incident happened.
Officers say the boy was chased and pulled from his bike by a white and brown "bull terrier" type dog.
A police spokesman said: "The boy was cycling along the road and went past a dog with a man thought to be the dog's owner.
"The dog is then reported to have come off its lead and chased the boy, pulling him from his bike.
"The boy was then bitten on the left knee and received a puncture wound injury.
"His injuries are thought to be minor and did not require stitching."
Police are now appealing for anyone with information, particularly the dog's owner, to get in touch.
The man thought to have been accompanying the dog is described as white, aged in his 40's with short brown hair. He was of average build and wearing a pink top and blue jeans.
Anyone with information should contact police on 101 quoting WA1716137.