The new sengi was discovered when camera traps were set out to assess fleeting visual sightings of a potentially unusual giant sengi. With the first photographs, it was clear that a new sengi had indeed been discovered, but actual specimens were needed to confirm this. A two-week expedition into the Ndundulu Forest was made in March 2006. The team of eight people was composed of four Tanzanians, two Americans, an Englishman, and the Italian expedition leader. On the 10 km hike into the area where the camera traps had been set, a partially eaten (probably by a raptor) sengi was very surprisingly found dead next to the trail - further confirming the discovery of a new species. Despite daily rain, and traps that turned out to be too small for the exceptionally large sengi, it took two weeks to capture four additional specimens with traditional snares. Between rain storms, the animals were photographed.

The four snared specimens were prepared in the field as museum study skins and then dried, but with great difficulty because of the soggy surroundings. The vouchers were then taken to The Natural History Museum (formerly the British Museum of Natural History) in London and compared with other specimens to confirm that the gray-faced sengi was indeed unique.

The new species is remarkably different than the golden-rumped sengi (R. chrysopygus), which is only found in coastal forests of Kenya, and the checkered sengi (R. cirnei), which is wide ranging in central and eastern African forests, including the Udzungwa Mountains below about 1000 meters. The gray-faced sengi superficially resembles the black and rufous sengi (R. petersi), which is found further to the north in the Eastern Arc Mountains and in coastal forests. None of the other giant sengis have a distinctive gray face with a jet-black lower rump. Also, the new sengi weighs about 700 g, compared to 300-500 g for the other three species.

The gray-faced sengi appears to be restricted to about 300 square kilometers of pristine wet montane forest in two separate areas. The Ndundulu-Luhomero area is protected by the West Kilombero Scarp Forest Reserve and Udzungwa Mountains National Park, whereas the Mwanihana area is entirely protected by the national park.

Although the size and color pattern of the gray-faced sengi are distinct from other giant sengis, preliminary observations suggest that many behaviors, including its diurnal activity, nesting, and foraging, are similar to the other three species.