Cucurbita fraterna, also known as Cucurbita pepo subsp. fraterna, is a mesophyte plant species of the genus Cucurbita. It is native to Tamaulipas and Nuevo León, Mexico. It has not been domesticated. It is the progenitor and nearest relative of the domesticated species Cucurbita pepo and wild C. pepo is still found in the same areas as C. fraterna. It was formally described by Liberty Hyde Bailey in 1943, in Gentes Herbarum.
Unlike most wild Cucurbita, some fruit specimens of C. fraterna have been found that were not bitter. Its usual habitat is dry upland scrub areas. It blooms in September and fruits ripen in December.
The debate about the relationship of C. pepo, C. fraterna, and C. texana have been going on since at least 1857. C. pepo has more similarities to C. fraterna than it does to Cucurbita texana which is also claimed to be an ancestor of C. pepo. Their isozymes are very similar. C. fraterna shares alleles with C. pepo at all 25 studied loci, indicating C. fraterna is C. pepo's nearest relative. C. pepo is most likely an early domesticated form of C. fraterna. It crosses well with both C. pepo and C. texana. C. pepo could be a compilospecies of C. fraterna and C. texana, which appear to be two species that were originally separate. Based on genetic allele analysis, there are two distinct groups within C. pepo: pumpkin, calabaza, criolla, and marrow squash are in one, and ornamental gourds, crookneck squash, acorn squash, pattypan squash, and a few others are in the other. C. fraterna is genetically closer to the first group and C. texana is genetically closer to the second group.