Nepenthes rajah (IPA pronunciation: /nəˈpɛn.θiz ˈrɑ.ʒə/) is an insectivorous pitcher plant species of the monotypic Nepenthaceae family. It is endemic to Mount Kinabalu and neighbouring Mount Tambuyukon in Sabah, Malaysian Borneo. N. rajah grows exclusively on serpentine substrates, particularly in areas of seeping ground water, where the soil is loose and permanently moist. The species has an altitudinal range of 1500 to 2650 m a.s.l. and is thus considered a highland or sub-alpine plant. Due to its localised distribution, N. rajah is classified as an endangered species by the IUCN and listed on CITES Appendix I.
N. rajah was first collected by Hugh Low on Mount Kinabalu in 1858. It was described the following year by Joseph Dalton Hooker, who named it after James Brooke, the first White Rajah of Sarawak. Hooker called it "one of the most striking vegetable productions hither-to discovered". Since being introduced into cultivation in 1881, N. rajah has always been a much sought-after species. For a long time, it was a plant seldom seen in private collections due to its rarity, price, and specialised growing requirements. Recent advances in tissue culture technology have resulted in prices falling dramatically, and N. rajah is now relatively widespread in cultivation.
N. rajah is most famous for the giant urn-shaped traps it produces, which can grow up to 35 cm high and 18 cm wide. These are capable of holding 3.5 litres of water and in excess of 2.5 litres of digestive fluid, making them probably the largest in the genus by volume. Another characteristic morphological feature of N. rajah is the peltate leaf attachment of the lamina and tendril, which is present in only a few other species.
N. rajah is known to occasionally trap vertebrates and even small mammals. Drowned rats have been observed in the pitchers of N. rajah. It is one of only two Nepenthes species documented as having caught mammalian prey, the other being N. rafflesiana. N. rajah is also known to occasionally trap other small vertebrates, including frogs, lizards and even birds, although these cases probably involve sick animals and certainly do not represent the norm. Insects, and particularly ants, comprise the majority of prey in both aerial and terrestrial pitchers.
Although N. rajah is most famous for trapping and digesting animals, its pitchers also play host to a large number of other organisms, which are thought to form a mutually beneficial (symbiotic) association with the plant. Many of these animals are so specialised that they cannot survive anywhere else, and are referred to as nepenthebionts. N. rajah has two such mosquito taxa named after it: Culex rajah and Toxorhynchites rajah.
N. rajah is known to hybridise relatively easily in the wild. Hybrids between it and all other Nepenthes species on Mount Kinabalu, with the exception of N. lowii, have been recorded. Due to the slow-growing nature of N. rajah, no hybrids involving it have been artificially produced as of yet.