Myrsine knudsenii is a small, branched tree or shrub standing 1.5 to 4.5 meters tall. The branches are pale to dark brown, tortuous, and glabrous. Its leaves are simple and alternate, with oblong-elliptic, elliptic, or elliptic-obovate blades measuring 4.5 to 8.5 cm long and 3 to 4.7 cm wide. The leaf apex is obtuse to rounded, often shallowly emarginate, and the base ranges from broadly cuneate to subtruncate. Leaves are dark green and dull when dry, with young ones appearing dark purplish-red and somewhat clustered toward the branch tips. They are coriaceous, with entire and revolute margins. The midrib is prominent, and lateral veins are moderately prominent, especially in dried specimens. Floral characteristics include 2 to 5 (up to 12 according to Hosaka) flowers in bracteolate fascicles below or occasionally among the leaves, with glabrous pedicels. The flowers are bisexual (perfect), with a calyx of 4 to 6 lobes imbricate or valvate in the bud and cream-colored petals narrowly oblanceolate, 3.5 to 4.2 mm long. The anthers are approximately 1.8 to 2 mm long, with a small tuft of hairs at the apex. The ovary is superior and 3 to 6-carpellate, leading to dark purple drupes (fruit) broadly ellipsoid to subglobose and approximately 5 to 8 mm in diameter. The seeds 1 per fruit; small, dark-colored, subglobose; excavated at the base. It is found in diverse mesic forests between 1,060-1,220 meters.