Nepenthes dubia /nɪˈpɛnθiːz ˈduːbiə/ is a tropical pitcher plant endemic to the Indonesian island of Sumatra, where it grows at an altitude of 1600–2700 m above sea level.

[2] Eleven years later, B. H. Danser formally described N. dubia in his seminal monograph "The Nepenthaceae of the Netherlands Indies".

[2][7] He wrote:[2] N. dubia strongly resembles the striking N. inermis, but the difference is too large to unite these two species.

[2] Bünnemeijer 938 was later designated as the lectotype of N. dubia by Matthew Jebb and Martin Cheek.

[5] Renewed interest in Nepenthes in the latter half of the 20th century saw N. dubia become the subject of both confusion and taxonomic revision.

In an article published in 1973 on the Nepenthes of Borneo, Singapore, and Sumatra,[4] botanist Shigeo Kurata incorrectly identified specimens of a natural cross between N. inermis and N. talangensis as belonging to N. dubia.

[9] In 1986, Mitsuru Hotta and Rusjdi Tamin included plant material belonging to N. dubia and N. inermis in their description of N. bongso.

In 1997, Matthew Jebb and Martin Cheek published their monograph "A skeletal revision of Nepenthes (Nepenthaceae)", in which they referred to N. dubia specimens from Mount Talang (Kurata s.n.

Charles Clarke restored N. tenuis to species rank in Nepenthes of Sumatra and Peninsular Malaysia (2001), citing differences in pitcher morphology.

It has an acute apex and is gradually attenuate towards the base, which clasps the stem for one third to a half of its circumference.

A pair of fringed wings (≤3 mm wide) runs down the upper third of the pitcher beneath the rim.

The inner surface of the pitcher is covered with numerous small, slightly depressed glands, occurring at a density of 600-900 per square centimetre;[2] it lacks a waxy zone.

Pitchers generally range in colour from light green to yellow throughout, although orange and red forms are also known to exist.

[2] Nepenthes dubia was for a long time thought to be endemic to Mount Talakmau, but it was recently found on a second mountain, where it grows at lower elevations of around 1600 m above sea level.

[8][12] The stunted upper montane forest which these species inhabit is dominated by ferns of the genera Dipteris and Dicranopteris.

[8] Nepenthes dubia produces thick, mucilaginous pitcher liquid similar to that found in related species such as N. inermis.

Nepenthes dubia belongs to a group of closely related montane Sumatran species that includes N. flava,[13] N. inermis, N. jacquelineae, N. jamban,[12] N. talangensis, and N. tenuis.

The lid of N. dubia is unique in this respect and is almost always reflexed beyond this angle, unless surrounding objects prevent it from assuming such a position.

[8] In their description of N. tenuis, Joachim Nerz and Andreas Wistuba included a table of morphological characteristics that distinguish it from related species, including N. dubia:[14] In 2001, Charles Clarke performed a cladistic analysis of the Nepenthes species of Sumatra and Peninsular Malaysia using 70 morphological characteristics of each taxon.

[15] In 2009, Adrian Y. Wartono observed a putative cross between N. dubia and N. jamban in an area where these two species grew with N. lingulata and N. rhombicaulis.

[11][17] Folia mediocria sessilia, lamina lanceolato-spathulata, nervis longitudinalibus utrinque c. 3, basi attenuata 1/3-2/3 caulis amplectente, vagina 0 ; ascidia rosularum et inferiora ignota ; ascidia superiora parva, parte inferiore tubulosa v. leviter ventricosa, supra medium infundibuliformia, costis 2 prominentibus ; peristomio fere horizontali, operculum versus acuto, applanato, 2-4 mm lato, costis 1/2-1/4 mm distantibus, dentibus 0 ; operculo anguste cuneato, facie inferiore plana ; inflorescentia ignota ; indumentum parcum, iuventute tomentum fuscum, denique deciduum.