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 dillenii 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 stricata, most in Opuntia dillenii).
References
Majure LC., W.S. Judd, P.M. Soltis & D.E. Soltis (2012) Cytogeographif the Humifusa clade of Opuntia s.s. Mill. 1754 (Cactaceae, Opuntioideae, Opuntieae): correltations with pleisocene refugua and morphological trais in a polyploid complex. Comparative Cytogenetics 6: 53-77.
Majure, LC., R. Puenee, M.P. griffith, W.S. Judd, P.M. Soltis, and D.E. Soltis. (2012) Phylogeny of Opuntia s.s. (Cactaceae): Clade delineation, geographic origins, and recticulate evolution. American Journal of Botany 99: 847-864.