Nepenthes rhombicaulis is known from a number of peaks in the Indonesian province of North Sumatra, particularly around Lake Toba. Plants that appear to match the description of N. rhombicaulis have also been recorded from Mount Bandahara in Aceh. The species is known with certainty only from the Mount Pangulubao complex and Mount Lubukraya, although Shigeo Kurata suggested that it is likely to be more widespread in the Lake Toba region. Nepenthes rhombicaulis has an altitudinal distribution of 1600-2000 m above sea level.