Geranium kikianum Kit Tan & G. Vold sp. nov. (Geraniaceae) is illustrated and described as a new species endemic to Greece. It occurs beside streams and other wet places in open Pinus nigra forest at the foothills of Mt Taigetos in the southern Peloponnese. It is closely related to G. macrorrhizum, a species more widely distributed in southern and central Europe, ranging from the southern Alps to the Balkan Peninsula. It differs from the latter by its smaller and narrower deflexed petal limb, slender claw and occurrence in wet habitats. The genome sizes of G. kikianum and of Greek populations of G. macrorrhizum are presented here for the first time. The nuclear DNA content (2C values) of 2.84 pg for G. kikianum and 2.83 and 2.87 pg for two G. macrorrhizum populations from Greece probably corresponds to a 2n=92 cytotype while the smaller genome size of 1.45 pg in a population of G. macrorrhizum from Mt Smolikas, Greece may correspond to a 2n=46 cytotype