Robert Jesty and Bethan Davies were abducted along with their driver on Friday while visiting Africa's oldest national park.
Mr Johnson paid tribute to the DRC authorities and the Congolese Institute for Nature Conservation for their "tireless help".
One added: 'Her only thought would have been the safety and security of visitors.' Last month, five young rangers and a driver were killed in a militia ambush at the park famed for its gorilla sanctuary.
The attack brought the number of rangers murdered in the park up to 175 in 20 years.
He said: "We wish to extend our sincerest condolences to her family, and our thoughts are with all those affected by this incident".
"I have received information that the two British tourists have been released".
A female wildlife ranger accompanying the pair was killed.
The Foreign Office had been in contact with local authorities since the event took place.
Cosma Wilungula, director general of the Congolese Institute for Nature Conservation, said: "We would like to thank our fearless team for ensuring the swift resolution of this incident, and the safe return of the two British nationals".
The park declined to say how the two Britons came to be released and if the kidnappers had been detained.
She was transferred to a nearby hospital to receive emergency medical treatment, but died shortly afterwards of her injuries, the park said.
It is home to to around a quarter of the world's critically endangered mountain gorillas and other endangered species as well as lions, elephants, hippos and a host of rare bird species.
The Foreign Office now - and before the kidnapping - advises against travelling to the area.