The otters "went crazy" after a runner passed the pack. Nipping at the heels of the park jogger, the startled animals descended on Spencer.
He assumes it was a case of mistaken identity after being startled by the runner.
Alarmed, the walkers ran to the nearby visitor centre and the bevy of otters gave chase.
After finding refuge in the visitor centre, the park guard returned and was able to offer first aid. However, Spencer took himself to hospital to have the bite wounds treated. The attack required $1300 of medical treatment.
"The whole process was 10 or 12 seconds, I couldn't move," Spencer told Reuters.
"I weighed more than 200 pounds and I couldn't get up without my friend's help, if the otters had attack a girl or child, they surely could not have survived,"he said.
Director of the Singapore Botanic Gardens, Dr Tan Puay Yok said he was aware of the incident.
"The Singapore Botanic Gardens' volunteers and staff monitor the movements of the otters and educate the public on the importance of observing them from a distance and not interacting with them," He told the Straits Times.
"In my years of documenting otters' behaviours, I have never heard of such an aggressive attack. Such attacks are not common" said Bernard Seah, a member of the Otter Working Group.
Animal rights group Otter City posted to their Facebook page saying that they were shocked to hear of the attack but believe to have footage of a jogger accidentally stepping on an otter.
"This is really very unfortunate for Mr Graham George Spencer, and we sincerely wish him a speedy and full recovery."
Smooth-coated otters grow up to 11 kg and are found throughout South East Asia.
While the otters have become a mascot for the garden city, tourists are warned not to feed or get too close to the animals and to always keep their dogs on a lead.
