Twenty-six year-old Erlend Berge Næsheim from Oslo had left the dock at Håøya north of Drobak when other sailors directed his attention to a dolphin nearby, reports, VG-Nyheter.
The dolphin took a reciprocal interest in Næsheim's 26-foot boat.
"It was huge, and quite wild," Næsheim told VG-Nyheter.
The sailor said that the ocean mammal then swam alongside him from Håøya to the top of Nesoddtangen - a distance of 18 kilometres in total.
The dolphin followed the boat north through the Oslofjord
Næsheim told VG-Nyheter that he had spotted dolphins in the fjord previously.
"I though this might have been the same one," said the 26 year-old.
"I thought it looked quite big... When it jumped out of the water, it waved its tail through the air. When the boat picked up speed, it got extra wild," recalled Næsheim.
Field biologist Viggo Ree told VG-Nyheter that the dolphin in the videos and pictures taken by Næsheim was rarely spotted in Norwegian waters.
"It is a short-beaked common dolphin. Only around 50 have ever been seen in Norwegian waters. Not long ago the total was no more than 10-15, so most have been spotted since the year 2000," Ree said.
The breed of doplhin spotted ny Næsheim is playful and social in character, Ree told VG-Nyheter.
"When they leave their pods and find their war to the North Sea or Oslofjord they still have their social antennas working, which means they are often drawn towards boats," the researcher continued.