"I pay tribute to the DRC authorities and the Congolese Institute for Nature Conservation for their tireless help during this awful case", he said.
A female park ranger, Rachel Makissa Baraka, was killed in the attack.
"My thoughts with the family of the ranger tragically killed during the kidnapping", Johnson wrote on Twitter.
Mr Johnson said: "I am delighted that two British nationals who were held hostage in the Democratic Republic of Congo have been released".
The 25-year-old ranger is the eighth to be murdered at the park this year.
The pair, whose release was arranged by guards at Virunga National Park, are uninjured and no ransom was paid, ITV News Africa Correspondent John Ray said.
Britain's Foreign Office said on Friday it was in contact with the Congolese authorities following an incident involving two British nationals and its staff were providing support to their families.
Cosma Wilungula, director general of the Congolese Institute for Nature Conservation, said: "We would like to thank our courageous team for ensuring the swift resolution of this incident, and the safe return of the two British nationals".
Virunga is dwelling to about one-quarter of the world's remaining mountain gorillas, and the work of defending them has confirmed harmful. More than 175 rangers have died protecting Virunga National Park, which is located in the rugged mountains and volcanic plains adjacent to neighboring Rwanda and Uganda.
Wildlife authorities have tried to protect it but 170 rangers have been killed over the last 20 years, he added. "Attacks could be indiscriminate", the Foreign Office wrote.
"Tourists in eastern DRC have been known to be left very vulnerable as a result of trying to travel independently without escorted transport, and the risk of kidnap or injury as a result of armed or criminal activity remains high".