The mammal, which appears to have suffered an injury near its tail fin, was discovered along the shores of Lough Foyle near Moville town centre about 2.30pm.
Locals managed to get the three-foot creature back to sea but high winds and strong waves washed it back into a nearby tributary, the Bredagh river.
Local man Anthony Craig, 55, found the porpoise as he walked his dog near his home at River Row, Moville.
“I was out walking about 2.30pm when the dog pulled me over towards the rocks. It seemed to be sniffing something out and when I went over to look there was a young porpoise lying there — it seemed in bad shape,” said Mr Craig.
“I ran to the house for a bucket and when we got back we started to throw buckets of sea water around him.
“He came around and got a bit of life about him then,” said Mr Craig.
“We then pushed him out into the sea but the winds were very high and the waves were too strong and kept washing him back in.
“The waves washed him into the Bredagh river and he has been moving up and down the river since about half-three today,” Mr Craig said.
Meanwhile, local student Rhea McLaughlin, who is studying to be a vet, got into the river to try and support the injured creature.
Mr Craig added: “He is trying to swim in a mixture of salt water from the Foyle and the river water so I don’t think it is the right environment for him. He seems to be getting weaker by the minute and he’s slowing down a good bit,” added Mr Craig.