Several Amazonian species of Plukenetia are remarkably rich sources of polyunsaturated fatty acids, in particular α-linolenic acid. The lipid composition of the large-seeded, recently described ‘Mountain Sacha Inchi’ Plukenetia carolis-vegae is reported here for the first time, and compared with Plukenetia huayllabambana, two cultivars of Plukenetia volubilis, and a newly developed hybrid between P. volubilis and P. carolis-vegae. All species and cultivars had a very high content of polyunsaturated fatty acids, 82.6–86.7% of total fatty acids, and at least 46.6% α-linolenic acid of total fatty acids. The highest content was found in P. carolis-vegae which had 57.4%. The exceptionally high α-linolenic acid content suggests that P. carolis-vegae may be an important plant-derived dietary source of this essential fatty acid and that the species has considerable potential for further domestication and commercialisation of its seeds and seed oil. A TAG analysis was carried out for the two P. volubilis cultivars, in which LLnLn and LnLL were most prevalent, and for P. huayllabambana, in which LLnLn constituted the largest fraction, followed by LnLnLn, indicating that this large-seeded species also has interesting dietary properties.