"All six crew members who were kidnapped from the oil tanker, Monjasa Reformer, are now safely recovered from an undisclosed location in Nigeria," Monjasa said.
It did not specify if ransom was paid.
Monjasa's CEO Anders Ostergaard said they "are in a relatively good health condition given the difficult circumstances they have been under in the last more than five weeks.
The Liberian-flagged Monjasa Reformer had 16 sailors on board when it was attacked by pirates on March 25. Despite being owned by a Danish company, none of the crew were Danish.
When the 135-meter-long tanker was found by the French navy the following week off the coast of Sao Tome and Principe in the Gulf of Guinea, the remaining crew said six of their colleagues had been kidnapped.
Monjasa had previously said it had lost contact with the crew after pirates boarded the ship.
Pirates have long been a risk in the Gulf of Guinea - a major shipping route stretching 5,700 kilometers from Senegal to Angola, with Nigerian gangs carrying out most attacks.
But since 2021, shippers say pirates have been raiding farther out in international waters.
Their violence and sophisticated tactics prompted pleas from shippers for a more robust foreign naval presence like the mission to curb attacks from Somali pirates a decade ago.
Many of the attacks in recent years have been carried out by Nigerian criminal gangs using speed boats from hideouts in the Delta region.
Two other attacks have been recorded in the region in 2023 so far.