"Nepenthes northiana" is very similar to "N. mapuluensis", a species known from only a handful of limestone peaks in East Kalimantan, on the other side of Borneo. Although there are few morphological characters separating these two taxa, there seem to be several stable differences that can be used to distinguish between them. Compared to "N. northiana", the leaves on the climbing stems of "N. mapuluensis" are more linear, the pitchers darker in colour, and the upper pitchers narrower. It is also worth noting that "N. northiana" is known only from the Bau area of Sarawak, which lies several hundred kilometres away from the only known populations of "N. mapuluensis".