According to local media, at least one man was taken to hospital on Thursday after encountering the sea mammal.
Japanese officials believe the same dolphin is responsible for at least six related attacks so far this summer.
While dolphins are not usually aggressive to humans, hostility towards swimmers is not unheard of.
Local media reported that one man was bitten on both his arms on Thursday morning, which was followed by another attack the same afternoon on a second man - who suffered injuries to fingers on his left hand.
The incidents both took place at Koshino Beach, a popular destination for swimmers and dolphins alike near the western Japanese city of Fukui.
Officials have now installed ultrasonic transmitters along the beach in the hope that its high-frequency noises will deter dolphins from the area.
Swimmers have also been warned of the dangers posed by the wild creatures - and have been instructed to avoid any in the water.
It is not currently clear what species of dolphin was involved in the attack.
According to local media, dolphins in the area have become used to human interaction and have been sighted in extremely shallow sections of the beach - including in knee-depth waters.
Worldwide, dolphin attacks are rare but not unheard of.
Scientists have suggested that wild bottlenose dolphins find swimming alongside humans "incredibly stressful," finding evidence that it disrupts their behavioural routines.
Photo file Caption: A bottlenose dolphin calf at six months old