The rarely seen event was witnessed by gig rowers in Newlyn Harbour in Cornwall.
Dolphin attacks on porpoises accounted for about one death a year on average, said Cornwall Wildlife Trust (CWT).
It is unclear why dolphins attack porpoises but it could be an aggressive response to feeding competition or even "misdirected sexual aggression".
More on the deadly attack, plus more Devon and Cornwall news
The pictures were captured by the Pendeen Pilot Gig Club in a training session.
CWT said: "We are unsure of why this happens and it is certainly not predation.
"Many theories have been put forward, including misdirected infanticide (bottlenoses will kill calves), misdirected sexual aggression or play behaviour and an aggressive response to feed competition are just some."
Marine Strandings Network said it found the carcass of the porpoise, which had "multiple severe injuries as a result of the attack".
"We do have a number of dead porpoise, and more unusually common dolphin, reported to us with injuries resulting from bottlenose dolphin aggression."
A bottlenose dolphin has been pictured flipping a porpoise into the air in a deadly attack.