Members of the public spotted the stricken animal on Saturday morning and kept it wet until specialists arrived.
The porpoise, a juvenile female, had suffered cuts and scrapes to its body, but after assessment by vets it was decided to attempt to refloat the youngster.
Niru Dorrian, an advanced marine mammal medic with British Divers Marine Life Rescue (BDMLR) and co-founder of marine conservation organisation Whalefish, took part in the rescue attempt. Experts from the Scottish SPCA animal welfare charity were also involved.
The team spent three hours in the sea supporting the porpoise and gently moving it in the hope it would recover sufficiently to swim away.
Unfortunately, efforts were in vain and the animal was euthanised on welfare grounds.
Mr Dorrian said: “We arrived on site after being alerted by the public to respond to a stranded harbour porpoise. When BDMLR medics and vets arrived, members of the public were already involved with the Scottish SPCA in keeping the animal wet.
“Upon arrival we assessed the animal, checked its body condition, vital signs and any kind of wounds and we made the decision to attempt to refloat it.
“We spent time carefully rocking the animal in order to restore its equilibrium, muscle movement and mobility.
“We spent about three hours in the water trying to get the animal to respond and to keep it comfortable, but in the end, sadly, we had to put it down.
“It was eventually agreed that the animal was unable to free swim and so the decision was made to put it to sleep.”
A necropsy will be undertaken by scientists from the Scottish Marine Animal Stranding Scheme in an effort to determine what caused the porpoise to become stranded.
Mr Dorrian thanked all those who assisted in the operation, adding: “The public were a great help, very responsive, assisting with the rescue and keeping the animal wet, which was a very positive thing to do.”
The latest incident comes just a week after a porpoise was successfully refloated in the Forth at Blackness after becoming stranded in a burn at Grangemouth.
Harbour porpoises are the smallest European cetacean species found in Scottish seas.
They are fairly common in coastal waters around the country and can often be spotted in the Forth.
Anyone who wants to know what to do when marine mammals washes up can find details on the BDMLR website.