Juvenile plants with internodes 2–5 cm long, 1.5 cm diam.; cataphylls to 12 cm long. Leaves with petiole to 29 cm long, sharply flattened laterally, 8–10 mm diam., margins thin; blade broadly ovate-cordate, 18–20 cm long, 13–15 cm wide, 1.3–1.45 × longer than wide, c. 0.6 × as long as petiole, abruptly long-acuminate at apex, prominently lobed at base; primary lateral veins 3–4 pairs; basal veins 6–8 pairs.
Adult plants with internodes 2–5 cm long, dark green, moderately glossy, short white-streaked near nodes; cataphylls moderately fleshy, unribbed near base, acutely Dshaped toward middle, soft, medium green, deciduous except for a thin layer of the base which may adhere to stem. Leaves with petiole 52–80 cm long, obtusely flattened (preadult petioles sharply D-shaped), dark green with even darker streaks; blade broadly ovate, deeply cordate-sagittate at base, abruptly acuminate at apex, 29–50 cm long, 25–42 cm wide, 1.1–1.15 × longer than wide, dark green and glossy above, moderately paler and matte below, drying dark grey-green and weakly glossy above, yellow-brown and matte below; margins broadly rounded; anterior lobe 21–36 cm long, margins broadly rounded; posterior lobes 12–20 cm long, 10– 18 cm wide; sinus obovate to narrowly hippocrepiform, 7–12 cm deep, 5–6 cm wide; midrib obtusely broad-flattened and concolorous above, convex below, drying concolorous above, slightly paler than surface below; primary lateral veins 5–6 pairs, arising at 45–50° angle, deeply sunken above, convex below; minor veins darker, fine, moderately distinct; basal veins 11 pairs, the 1st and 2nd pair free to the base, 5th and higher order veins coalesced 2.5–3 cm into a posterior rib. Inflorescences 1–3 per axil; peduncle 6.5–8 cm long, pale green, coarsely white-streaked near apex; spathe 13–14 cm long, 2.5 cm diam. when furled, semiglossy, tube flattening to 10 cm wide, maroon outside, except pale green along margins, dark maroon and glossy inside, drying dark brown; blades medium green tinged purplish violet along margins outside, and throughout much of the lower half of white inner surface, the constricted area flattening to 3 cm wide; spadix 10.5–13.3 cm long; pistillate portion 5 cm long, drying 10–11 mm diam., greenish-white; staminate portion 7–8.5 cm long, drying 11–12 mm diam., cream; pistils 2.4–2.6 mm long, 2.2–2.4 mm diam., locules 3–4; style 1.2–1.5 mm diam., drying light yellow-brown, conspicuously granular, stigma 3.5–4.2 mm diam., drying blackened with a distinct depression medially; ovules 1 per locule, basal; seeds 1.7–1.2 mm long, 0.8–1 mm diam., medium-dark yellow-brown, one end with a somewhat constricted nipple.
It is apparently a member of Philodendron subg. Philodendron sect. Macrobelium subsect. Glossophyllum series Ovata and is characterised by its sharply flattened juvenile petioles, sharply D-shaped pre-adult petioles, obtusely flattened adult petioles, broadly ovate-cordate blades which are glossy above and matte below. It differs from most species in that section by the glossy upper surface of the leaf blades. Philodendron devianum is somewhat similar to P. merenbergense (also published in this article), but it differs by having larger leaves, which are more deeply lobed and have a distinct posterior rib, and by its much larger inflorescences (spathe 13–14 cm long in P. devianum versus 5–6 cm long in P. merenbergense).