The shocking discovery was made near Leighswood Road in Walsall by a member of the public.
It has prompted an appeal from the RPSCA to find out what happened to the animal, which wasn't microchipped, on January 27.
Inspector Nayman Dunderdale said: “This is a very sad discovery and we’d like to thank the member of the public who found him.
“The dog – who is thought to be around six to seven years old - was found with injuries, which included numerous superficial grazes/lacerations over the head, thorax and hindlimbs.
“Findings from the vet suggest trauma prior to the time of death, but there was no evidence of fractures which would further support the cause of the trauma. This also meant there was also no obvious cause of death.”
The RSPCA is appealing for information on the "suspicious" incident and is urging anyone who can help with enquiries or who may know the dog's owner to call the charity’s appeal line on 0300 123 8018.
Inspector Dunderdale added: “He was found in a secluded spot so there wasn’t any CCTV, and it seems unlikely that the dog jumped or accidentally fell from the bridge. Although the evidence doesn’t support that he was thrown from the bridge, we don’t know how he got there, but it appears he was dumped.
“We’d appreciate anyone with information that could assist with our enquiries to please get in touch.”