Marcellus of Side was a Greek physician and didactic poet of the 2nd century CE, active during the reign of Marcus Aurelius. A native of Side in Pamphylia, he dedicated his major poetic work to that emperor.

His only known work is On Medicine, a didactic medical poem in 42 books composed in dactylic hexameter and addressed to Marcus Aurelius. The poem is almost entirely lost, surviving only in fragments and testimonia; one extant fragment details the medicinal uses of the peony plant.

Marcellus represents the continued tradition of scientific didactic poetry in the Imperial period, following figures like Nicander. The scale of his lost work and its imperial dedication underscore the cultural prestige of such scholarly verse. His fragments remain valuable for the study of ancient pharmacology and literary practice.