Nepenthes neoguineensis is a climbing plant.
The stem is up to 6 mm thick and cylindrical to obtusely angular in cross section, especially in the upper part of the internodes.
Internodes are up to 4 cm long.

Leaves are chartaceous and petiolate.
The lamina is lanceolate and reaches 25 cm in length and 4.5 cm in width.
It has an acute apex and is gradually or rather abruptly attenuate towards the base.
Three or four indistinct longitudinal veins are present on either side of the midrib.
Pinnate veins ascend obliquely from the midrib and are irregularly reticulate in the outer part of the lamina.
Tendrils are up to 4 mm thick and about as long as the lamina.

Upper pitchers gradually arise from the ends of the tendrils, forming a 7 to 23 mm wide curve.
They are infundibulate in the lower part, somewhat ventricose at two-fifths of their height, and widened towards the mouth.
A pair of fringed wings (≤8 mm wide) runs down the entire length of the pitcher.
The pitcher mouth is oblique and acuminate towards the lid.
The peristome is cylindrical or flattened and up to 1.5 mm wide.
The glandular region covers the lower third of the inner surface of the pitcher.
The glands occur at a density of about 700 to 900 per square centimetre.
The lid is suborbicular, truncate or slightly emarginate, and rounded or slightly cordate at the base.
Round, depressed glands are present on the undersurface of the lid, being concentrated and increasing in size towards the middle.
A flattened, unbranched spur (≤3 mm long) is inserted at the base of the lid.

The male inflorescence is a long, cylindrical panicle.
The peduncle reaches 12 cm in length and 4 mm in width.
The rachis is attenuate and up to 44 cm long.
Pedicels lack bracteoles, reach 55 mm in length, and are one- to four-flowered.
Tepals are orbicular-elliptical and around 4 mm long.
Stamens are about 4 mm long, including the anthers.

The female inflorescence is a panicle-like raceme.
The peduncle may be up to 15 cm long and 2.5 mm wide.
The rachis is attenuate and reaches 20 cm in length.
Pedicels are up to 35 mm long and most have filiform bracteoles.
They are one- to three-flowered.
Tepals are oblong-lanceolate and approximately 4 mm long.
The ovary is sessile.

Nepenthes neoguineensis has a very sparse indumentum.
The stem is virtually glabrous, as is the lamina.
Tendrils are densely hirsute when young, becoming only hairy near the pitcher or entirely glabrous when mature.
Pitchers have a dense covering of caducous stellate hairs.
The exception is the spur, which has persistent stellate hairs.
Inflorescences have a very dense indumentum of short, white or brownish stellate hairs.
The pedicels, tepals and the ovary are very densely stellate-tomentose.

Lower pitchers range in colour from light green to dark purple throughout.
Upper pitchers are generally yellowish-green, often with a darker peristome and bright red lid.
Herbarium specimens are fallow-dun in colour.
Nepenthes neoguineensis is a climbing plant. The stem is up to 6 mm thick and cylindrical to obtusely angular in cross section, especially in the upper part of the internodes. Internodes are up to 4 cm long.
Leaves are chartaceous and petiolate. The lamina is lanceolate and reaches 25 cm in length and 4.5 cm in width. It has an acute apex and is gradually or rather abruptly attenuate towards the base. Three or four indistinct longitudinal veins are present on either side of the midrib. Pinnate veins ascend obliquely from the midrib and are irregularly reticulate in the outer part of the lamina. Tendrils are up to 4 mm thick and about as long as the lamina.
Upper pitchers gradually arise from the ends of the tendrils, forming a 7 to 23 mm wide curve. They are infundibulate in the lower part, somewhat ventricose at two-fifths of their height, and widened towards the mouth. A pair of fringed wings (≤8 mm wide) runs down the entire length of the pitcher. The pitcher mouth is oblique and acuminate towards the lid. The peristome is cylindrical or flattened and up to 1.5 mm wide. The glandular region covers the lower third of the inner surface of the pitcher. The glands occur at a density of about 700 to 900 per square centimetre. The lid is suborbicular, truncate or slightly emarginate, and rounded or slightly cordate at the base. Round, depressed glands are present on the undersurface of the lid, being concentrated and increasing in size towards the middle. A flattened, unbranched spur (≤3 mm long) is inserted at the base of the lid.
The male inflorescence is a long, cylindrical panicle. The peduncle reaches 12 cm in length and 4 mm in width. The rachis is attenuate and up to 44 cm long. Pedicels lack bracteoles, reach 55 mm in length, and are one- to four-flowered. Tepals are orbicular-elliptical and around 4 mm long. Stamens are about 4 mm long, including the anthers.
The female inflorescence is a panicle-like raceme. The peduncle may be up to 15 cm long and 2.5 mm wide. The rachis is attenuate and reaches 20 cm in length. Pedicels are up to 35 mm long and most have filiform bracteoles. They are one- to three-flowered. Tepals are oblong-lanceolate and approximately 4 mm long. The ovary is sessile.
Nepenthes neoguineensis has a very sparse indumentum. The stem is virtually glabrous, as is the lamina. Tendrils are densely hirsute when young, becoming only hairy near the pitcher or entirely glabrous when mature. Pitchers have a dense covering of caducous stellate hairs. The exception is the spur, which has persistent stellate hairs. Inflorescences have a very dense indumentum of short, white or brownish stellate hairs. The pedicels, tepals and the ovary are very densely stellate-tomentose.
Lower pitchers range in colour from light green to dark purple throughout. Upper pitchers are generally yellowish-green, often with a darker peristome and bright red lid. Herbarium specimens are fallow-dun in colour.
Nepenthes neoguineensis