Nepenthes rajah is a carnivorous pitcher plant species of the family Nepenthaceae. It is endemic to Mount Kinabalu and neighbouring Mount Tambuyukon in Sabah, Malaysian Borneo. Nepenthes 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 1,500–2,650 m (4,920–8,690 ft) above sea level 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.

The species was collected by Hugh Low on Mount Kinabalu in 1858, and described the next year by Joseph Dalton Hooker, who named it after James Brooke, the first White Rajah of Sarawak. Since being introduced into cultivation in 1881, it has always been a sought-after species, although costly and hard to cultivate. Tissue culture has allowed it to become more widespread in cultivation.

N. rajah is best known for the giant urn-shaped traps it produces, which can grow up to 41 cm (16 in) high and 20 cm (7.9 in) wide. These are capable of holding 3.5 L (0.92 US gal) of water and in excess of 2.5 L (0.66 US gal) of digestive fluid, making them probably the largest in the genus.

N. rajah can trap mammals as large as rats. N. rajah occasionally traps small vertebrates such as frogs, lizards, and even birds. Insects, and particularly ants, are its staple prey. The pitchers are host to many other organisms, some so specialised that they cannot survive anywhere else, and are called nepenthebionts. N. rajah has two such mosquito taxa named after it: Culex rajah and Toxorhynchites rajah. The species is able to hybridise in the wild with all other locally occurring Nepenthes species.

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