The incident happened at around 11.45am on Thursday, March 9, in the Brynhafod park in Maserfield, Oswestry.
The dog owner is described as being a woman in her 40s, around 5ft 6ins tall, with shoulder length dark blonde hair. She was said to be warning a long, dark-coloured puffa coat and carrying a yellow hiking pole, and in company with several children.
The boy was bitten by a small light brown dog, possibly a chihuahua or similar, which was off the lead at the time.
Pc Melanie Davies said: “We would like to speak to the dog owners(s) so we can establish exactly what happened.
"The boy received a puncture wound to his leg. He was shaken by the incident and was given a tetanus injection."
The owner of the dog or anyone with any information is asked to get in touch with PC Davies on 101 extension 770 4031 or email her direct: firstname.lastname@example.org.