The bird was found badly hurt in Colney Heath Lane in St Albans at the beginning of the month.
Injuries to his eyes, chest and legs were "indicative of the barbaric blood sport of cockfighting", said the animal charity.
The bird - named Levi Rooster - is now able to see again after treatment.
Irene Hansford, chairman of the RSPCA Hillingdon branch said: "We all felt sorry for Levi when he arrived.
"But thanks to the veterinary and animal welfare team at the RSPCA Hillingdon Clinic, who fought very hard with constant treatment and tit bits of food to help him eat and restore him to health.
"He started to eat and we were delighted to hear him start crowing. The final reward was when one of his eyes opened following, a few days later, with the second."
Ms Hansford added that once Levi is back to full health he will be found a new home.
RSPCA inspector Jaime Godfrey said: "Investigations are ongoing in this case and we are working alongside Hertfordshire Constabulary."
RSPCA inspector Mike Butcher believes Levi had been used in "an organised fight".
His team specialises in tackling so-called "blood sports" such as cockfights, which have been illegal since 1835.
"This cockerel's spur had been cut off. This was probably to fit a metal spur so it could inflict maximum damage to the opponent's bird during a cockfight," he said.
The charity has appealed for information about who may have abandoned him.
A cockerel blinded from having his eyes pecked in a suspected cockfight has regained its sight after it was found and cared for by the RSPCA.