Rubus × 'Merton Thornless' is a historically pivotal blackberry cultivar released in 1935 by the John Innes Horticultural Institution in Merton, England. It originated from a sophisticated genetic cross between 'Rubus ulmifolius var. inermis' (a naturally thornless but small-fruited wild species) and 'John Innes' (a vigorous, thorny hybrid of Rubus ulmifolius and Rubus thrysiger (now named Rubus hastiformis). This hybridization was a scientific breakthrough; because the thornless parent was diploid and the thorny parent was tetraploid, breeder M.B. Crane had to utilize chemically induced polyploidy to create a fertile, tetraploid offspring. This allowed the recessive thornless trait to be stabilized in a plant that finally produced large, commercially viable fruit, effectively serving as the genetic "ancestor" for many modern thornless blackberry varieties.

The fruit of 'Merton Thornless' is large, round, and exceptionally juicy, with a sweet, mild flavor that lacks the intense astringency of many wild brambles. The plant exhibits a vigorous, trailing growth habit with thick, smooth canes that can exceed 10 feet in length, necessitating a robust trellis system. Gastronomically, it is valued for its late-summer ripening window and its high juice yield. While it has been largely superseded by more erect cultivars in commercial farming, it remains a cornerstone of ethnobiological study for its role in the domestication and genomic engineering of the Rubus genus.