Evergreen trees or shrubs, mostly small (< 10 m), rarely up to 30 m tall.
Branchlets 5 cm below terminal bud ca. 1-2.5 mm in diam., glabrous to sparsely pubescent with short (< 0.4 mm), appressed trichomes. Terminal vegetative buds conical to elongate, minute or up to 8 mm long and 1.5 mm thick, at least their apical part densely covered with short, straight, appressed trichomes.
Petioles 2.5-12 mm long, ca. 1-1.5 mm thick, glabrous or nearly so. Leaves alternate, usually (ob)lanceolate, widest 1/3-2/3 from the base, mostly at the middle, (3.5-) 5-12.5 (-16) cm long, 1.2-3 (-4) cm wide, (2.5-) 3-6 (-7.5) times longer than wide, tip (usually narrowly) acuminate, sometimes tapering towards a rather indistinct acumen, base attenuate to narrowly acute, margin usually flat, rarely very narrowly recurved, midrib lighter than the lamina, mostly slightly convex above, more distinctly raised below, secondary veins slightly raised to relatively inconspicuous, 5-12 (-14) on each side of the midrib, diverging at 40-70°, tertiary venation reticulate, inconspicuous. Indument (if present) consisting of short (< 0.4 mm), appressed trichomes, sparse on both sides to glabrous from the beginning, axils of secondary veins on lower leaf surface usually without domatia, in some populations with erect hairs.
Inflorescences in the axils of cataphylls, usually along young growth below the terminal bud, sometimes also on axillary brachyblasts (the vegetative bud occasionally growing through, leaving the inflorescences at its base), almost never in the axils of foliage leaves, slender (< 1 mm in diam. at the base), ca. 2-12 cm long, lateral branches (0-) 2-8 (-12) below terminal cyme or cluster of cymes, branched 0-2 (-3) times, indument consisting of short (< 0.4 mm), appressed to ascending trichomes, moderately to very sparse, occasionally up to dense on receptacle. Pedicels 1-4 (-8) mm long, <0.5 mm thick.
Flowers (4-) 5-6 (-9) mm in diam., tepals mostly oblong with almost parallel sides, ca. 1.5-3.5 mm long, ca. 1-1.5 mm wide, covered with fine papillae on the adaxial side. Stamens ca. 1 mm long, with a usually very short filament, anthers weakly papillose to nearly glabrous, in the two outer whorls transverse elliptic to almost squarish, apically very broadly obtuse to slightly emarginate, sometimes slightly apiculate, in the third whorl more or less rectangular, apically usually roundish-truncate. Staminodes reaching ca. half the length of the stamens, terete to slightly clavate, usually with a glandular tip, free or fused with the inner stamens at the base. Pistil ca. 1.5-2 mm long, glabrous, ovary ellipsoid to subglobose, style about as long as the ovary. Receptacle bowl-shaped to cup-shaped, often somewhat closed above by the bases of stamens and staminodes, glabrous or with a few appressed hairs inside.
Fruit ellipsoid, ca. 9-12 mm long, ca. 6-10 mm in diam., cupule mostly shallowly bowl-shaped, ca. 1-2.5 mm deep and ca. 5-8 mm in diam., pedicel slightly to distinctly thickened towards the cupule.
Southern Minas Gerais (Brazil) to Uruguay, in usually moist forests from near sea level to about 1000 m altitude.
Nectandra megapotamica is recognized by its narrowly lanceolate leaves (somewhat willow-like) and its (for Nectandra) relatively small flowers. It is one of the most frequent species in the area, especially in gallery forests.