Plants to 2 m tall, shrubby, sprawling or erect. Stem segments flattened, 10-25(-40) cm long, 5-15(-25) cm wide, not disarticulating, dull to bluish-green, narrowly elliptic or obovate, tuberculate, making margins appear scalloped between raised areoles, glabrous; areoles 3-5 per diagonal row across midstem segments, 3-6.5 mm long, 3.5 mm wide, oval; wool dense, tan; spines 1-11 per areole in most areoles, the longest stout, oval in cross section, 12-40(-60) mm long, not markedly barbed, spreading in all directions, yellow, aging brown, straight or curving; glochids to 4 mm long, inconspicuous, few to many in crescent at adaxial edge of areole, yellow, aging brown, often incurved, subequal to increasing in length toward adaxial edge of areole. Flowers 5-6 cm wide: inner tepals 25-30 mm long, lemmon yellow throughout when mature, sometimes reddish when immature; filaments yellow; anthers yellow; style and stigma lobes yellowish. Fruits 40-60 mm long, 25-30(-40) mm wide, purplish throughout, stipitate, ellipsoid or barrel-shaped, juicy, spineless; areoles 6-10; seeds tan, subcircular, 4-5 mm long, 4-4.5 mm wide, with slightly irregular surface; girdle protruding to 1 mm. 2n= 44 (cultivated), 66.
Opuntia stricta and Opuntia tuna are sometimes considered to refer to different parts of the variation within a single species, "stricta" referring to plans with few spines. Cytological and molelulcar research supports their treatment as different species that differ in the abundance of areoles with well-developed spines (few in Opuntia stricta, most in Opuntia tuna). Note: Guigi & Marriotti (2022) showed that the species that had been known as Opuntia dillennii was the same as Opuntia tuna. Because the name Opuntia tuna was published before Opuntia dillenii, it becomes the correct name of the species.