It was found on Saturday evening on the shoreline near Lowestoft.
Coastguards believe the mammal - in an advanced state of decomposition - may have been struck by a boat.
The Maritime and Coastguard Agency has passed on details to the Receiver of Wreck.
It is hoped the carcass will refloat at high tide.
"On arrival and investigation it was found to be a harbour porpoise approx 5ft long," a statement from HM Coastguard Lowestoft and Southwold said.
It was deemed the mammal "had been struck by a vessel's prop, potentially in and around the Lowestoft Harbour."
Porpoises are smaller than dolphins and lack the distinctive beak.
The harbour species is so-called because it is often found in more shallow coastal areas and estuaries.
Three whales have been discovered beached along the nearby coastline in recent days.
The first, a 20ft minke calf, was spotted on a mudbank at Orford, on 20 May.
An area close to Felixstowe Pier was sealed off on 21 May after the decomposed body of a 30ft fin whale washed up.
The third, believed to be the remains of a sperm whale, was spotted off the coast on Monday and is thought to have floated back out to sea.
Scientists from the Cetaceans Strandings Investigation Programme (CSIP) said those three deaths were "coincidental" and not thought to be in any way linked.
A harbour porpoise has been found washed up on the Suffolk coast - the fourth marine mammal to have been found dead in a little over a week.