The pair are understood to have been taken while visiting the Virunga National Park, a renowned gorilla sanctuary in the east of the African country, on Friday.
Two British tourists are reported to have been taken hostage in the Democratic Republic of Congo. The ranger was killed, and the driver and two tourists were among those abducted.
Joel Wengamulay, a spokesman for the Congolese Institute for The Conservation of Nature, told the United Nations-backed Radio Okapi that investigations into the attack have begun.
The Foreign and Commonwealth Office advises against all but essential travel to DRC capital Goma and has urged Britons not to go beyond the city.
Ms Katumwa is understood to be the first female ranger to be killed in the park, which has been riven by violence in recent years.
The national park, which runs along the border with Uganda and Rwanda, covers 3,000 sq miles (7,800 sq km).
Congolese soldiers and park rangers are searching for two British tourists kidnapped in the country's volatile eastern borderlands, an army spokesman said.
It is a Unesco world heritage site and is home to critically-endangered mountain gorillas as well as lions, elephants and hippos.
In an interview with National Geographic a year ago, Virunga National Park Director Emmanuel de Merode said three main groups are responsible for the violence in the park.
But it is located in DR Congo's North Kivu province, where armed groups are fighting for control of territorial and natural resources, and poaching is a major threat.
On April 9, five rangers and a driver were killed in an ambush in the park, which was established in 1925. Park officials blamed Mai Mai militia members.