Alkali grasses (Puccinellia) are temperate and arctic grasses of coastal and alkaline habitats, with ploidy levels that range from diploid to octoploid. This paper investigates the species limits of diploid alkali grasses in the North American Arctic. We used flow cytometry to confirm that four to seven of the 13–19 initially recognized taxa in the North American Arctic are diploid. Multivariate analyses of both morphological and amplified fragment length polymorphism (AFLP) data were congruent in resolving five or six diploid species: 1) P. arctica of northern and western distribution, including P. agrostidea and P. poacea; 2) a new diploid species from Banks Island, N.W.T., Canada; 3) P. tenella subsp. langeana; 4) P. alaskana, which had been previously treated as a subspecies of P. tenella; and 5) circumpolar (except coastal Beringia) P. vahliana, from which 6) the coastal Beringian endemic P. wrightii is distinguishable only on size and for which subspecies status may be more appropriate. In common garden experiments, eleven of 21 quantitative morphological characters varied significantly between field and common garden, showing phenotypic plasticity that explains much of the difficulty in identification. We present a map showing known geographic ranges of diploid Puccinellia species in the North American Arctic.