Nepenthes thorelii is closely related to the Indochinese species N. smilesii.
These two taxa share several morphological features, such as narrow linear leaf blades and leaf bases that clasp the stem.
In addition, N. smilesii may grow in similar habitats to N. thorelii.

Distinguishing features of N. thorelii include the large rootstock that allows it to perennate through dry and wet seasons, the uncurled tendrils of upper pitchers, and its large inflorescence.
However, the extent of the variation of the Indochinese species is unknown, making them difficult to circumscribe.

Nepenthes thorelii is also allied to N. bokorensis of Cambodia, from which it differs in several aspects of vegetative morphology.
Firstly, the lamina of N. bokorensis is sessile to sub-petiolate and only slightly decurrent down the stem, if at all.
In comparison, N. thorelii has an amplexicaul leaf attachment and the base of the lamina is decurrent into two wings that extend up to 2.5 cm down the stem.
In addition, N. thorelii has wholly ovoid lower pitchers, whereas those of N. bokorensis are only ovate in the basal third, becoming cylindrical above.

In his Carnivorous Plant Database, taxonomist Jan Schlauer treats N. suratensis as a possible heterotypic synonym of N. thorelii.
Nepenthes thorelii is closely related to the Indochinese species N. smilesii. These two taxa share several morphological features, such as narrow linear leaf blades and leaf bases that clasp the stem. In addition, N. smilesii may grow in similar habitats to N. thorelii.
Distinguishing features of N. thorelii include the large rootstock that allows it to perennate through dry and wet seasons, the uncurled tendrils of upper pitchers, and its large inflorescence. However, the extent of the variation of the Indochinese species is unknown, making them difficult to circumscribe.
Nepenthes thorelii is also allied to N. bokorensis of Cambodia, from which it differs in several aspects of vegetative morphology. Firstly, the lamina of N. bokorensis is sessile to sub-petiolate and only slightly decurrent down the stem, if at all. In comparison, N. thorelii has an amplexicaul leaf attachment and the base of the lamina is decurrent into two wings that extend up to 2.5 cm down the stem. In addition, N. thorelii has wholly ovoid lower pitchers, whereas those of N. bokorensis are only ovate in the basal third, becoming cylindrical above.
In his Carnivorous Plant Database, taxonomist Jan Schlauer treats N. suratensis as a possible heterotypic synonym of N. thorelii.
Nepenthes thorelii