Terrestrial [description from dried material]. Internodes short, 1-1.5 cm long, drying dark black, closely ribbed longitudinally. Cataphylls green, unribbed, (5.5-)7-13.5 cm, 1 or 2 persisting at upper nodes. Leaves erect with pendent blades; petioles subterete, 11-43 cm long (averaging 27.7 cm), drying black; blades triangular-ovate-subsagittate, drying dark brownish black above, paler and dark greyish below, 18-38 cm long (averaging 28 cm), 5-22 cm wide (averaging 14.5 cm), 1.9 times longer than wide, shallowly to moderately lobed at base; anterior lobe 13-31 cm long (averaging 24.4 cm); posterior lobes 2.5-12 cm long (averaging 7.4 cm); sinus usually arcuate to parabolic, rarely V-shaped, 0.5-8 cm deep (averaging 3.6 cm); midrib flattened and concolourous above, drying darker than surface above, reddish brown and darker than surface below; midrib narrowly rounded and slightly paler above, narrowly rounded and paler below; basal veins 2-5 (averaging 3), usually 1st and sometimes 2nd basal vein free to the base, 2nd-3rd and 4th usually fused for 1.3 cm; posterior rib not at all naked or sometimes weakly naked near petiole; primary lateral veins 3-8 pairs per side (averaging 5), departing midrib at 35-50° angle. Inflorescence 1-2 per axil; peduncle 6-10 cm long, drying 0.2-0.3 cm in diam., dark brown, matte. Spathe (4-)5.2-7.5 cm long, tube green, ellipsoid, (2.5-)3-4 cm long; blade white with reddish purple spots, (1.5-)2-4.5 cm long. Spadix to 6.7 cm long, 0.6-0.7 cm stipitate; pistillate portion 1.3 cm long, 0.5-0.6 cm in diam. on drying; staminate portion 4-4.7 cm long, 1.7 cm in diam. on sterile staminate segment, to 1.7 cm in diam. at constricted area. Ovary 5 mm long, 4-5-locular; ovules 1-2 per locule, attached basally, surrounded by a broad envelope; funicle about as long as the ovule, attached about midway on the ovule. Infructescence with peduncle to 14 cm long, spathe tube to 5.2 cm long, 3.5 cm in diam. Seeds c. 8-10 per locule, cylindrical, drying dark brown, 1.5 mm long, 0.5 mm in diam., with both sides enveloped by a spongy appendage extending along the entire edge and making the seeds 1-1.2 mm wide with the opposite sides of the seeds exposed, the ends usually overtopped by the spongy appendage.
Planta terrestris; internodia brevia, 1-1.5 cm diam.; cataphylla viridia, (5.5-)7-13.5 cm longa, decidua; petiolus 11-43 cm longus; lamina triangulari-ovato-subsagittata, 18-38 cm longa, 5-22 cm lata; lobulis posterioribus 2.5-12 cm longis, nervis primariis lateralibus 3-8 utroque; inflorescentia 1-2 per quoque axillam, pedunculo 6-10 cm longo; spatha (4-)5.2-7.5 cm longa, tubo viride, lamina alba; spadix 6.7 cm longus.
Philodendron rimachii is endemic to the Loreto Department in Peru, along the Amazon River between Iquitos and Yanomono at 90-150 m elevation, in mature tropical moist forest (Holdridge 1967) along small streams on mud, in swamps over clay or white sand, or in mature upland forest on poorly drained, sandy or lateritic soils.