Terrestrial. Internodes 1-2 cm long, 2-2.5 cm in diam., dark green and weakly glossy becoming grey-brown. Cataphylls 17-19 cm long, sharply 2-ribbed, green, drying dark brown and coriaceous, deciduous. Leaves erect with pendent blades; petioles 25-42 cm long (averaging 33.6 cm), dark green and weakly glossy, obtusely flattened adaxially; blades narrowly ovate-semisagittate, 32-37 cm long (averaging 33.31 cm), 7-21 cm wide (averaging 16.7 cm), 1.7-2 times longer than wide, abruptly long-acuminate at the apex, weakly lobed at base, inequilateral with one side up to 1.5 cm wider, subcoriaceous, dark green and semiglossy above, moderately paler and weakly glossy below, drying weakly brown and ± matte above, dark yellowish brown and moderately glossy below; anterior lobe 29-33.5 cm long (averaging 30.81 cm), the posterior lobes rounded to broadly rounded, 4-7 cm long (averaging 5.76 cm), 3-6 cm wide (averaging 4.3 cm); sinus widely parabolic to arcuate, (0.8-)1.5-4 cm deep (averaging 2.5); major veins drying paler than upper surface and darker than lower surface; margins weakly undulate; basal veins in 4 pairs, all free to the base or with the lowermost briefly united for 1-1.5 cm; posterior ribs absent or when present naked for up to 1 cm; midrib flat and slightly paler above, convex, darker and densely short-red-lineate below; primary lateral veins 6-9 pairs per side, sunken and concolourous above, maroon and convex below (at least toward base), departing midrib at 50° angle, drying slightly darker or slightly paler than the surface; interprimary veins moderately distinct; conspicuously flat above and below; minor veins moderately distinct on fresh leaves but rather inconspicuous on drying, alternating with short-lineate or punctiform secretory ducts. Inflorescences (post-anthesis) 2 per axil; peduncle green, 2-3 cm long. Spathe 10 cm long, 1.8 cm in diam. on tube, cuspidate at apex, medium green on tube outside, paler within, tinged faintly pink on tube inside. Spadix 6.3-11 cm long; pistillate portion 5.6-7.8 cm long, 0.7-1.1 cm in diam. midway, 0.6 cm in diam. at base, 0.3-0.9 cm in diam. at apex; staminate portion 3.2 cm long, drying 0.5-0.6 cm in diam., protruding well beyond the end of the spathe after anthesis. Pistils closely compacted, 5 mm long; style 1.2-1.6 mm wide, irregularly rounded, the surface drying granular, the margins thin, stigma rounded 0.2-0.4 mm in diam., depressed-globose, button-shaped on drying with a weak depression in the middle; ovary 4-5-locular; juvenile seeds narrowly ovoid to ovoid-ellipsoid, 1 mm long, finely striate longitudinally, 1-2 per locule, one positioned near the base and one closer to the apex.
Planta terrestris; internodia 1-2 cm longa, 2-2.5 cm diam.; cataphylla 2-costata, decidua; petiolus 25-42 cm longus; lamina anguste ovato-semisagittata, 32-37 cm longa, 7-21 cm lata, lobulis posterioribus 4-7 cm longis, 3-6 cm latis, nervis primariis lateralibus 6-9 utroque; inflorescentia 2 per quoque axillam, pedunculo viridi, 2-3 cm longo; spatha 10 cm longa, 1.8 cm diam., tubo viridi extus, subroseo intus, 6.3-11 cm longo.
Philodendron huaynacapacense ranges from central Peru to N Bolivia at 600-1294 m. It is currently known only from two localities. The type collection was made in Peru in the Huanuco Department in a premontane wet forest life zone Holdridge 1967) at about 1300 m elevation, but a second collection was made more than 1000 km further south in Bolivia at 600 m in the Department of Cochabamba.