Nepenthes rhombicaulis was first collected by Shigeo Kurata on March 29, 1972, on Mount Pangulubao at an altitude of between 1700 and 1900 m above sea level. The species was mentioned by name in a 1972 issue (volume 26, number 10, page 44) of The Heredity. It was formally described by Kurata the following year in The Gardens' Bulletin Singapore. One of the original specimens, Kurata 4300, was designated as the holotype of the species and is deposited at the herbarium of the Nippon Dental College (NDC). An isotype is held at the National Herbarium of Singapore (SING).