Abstract. Eleocharis cryptica (Cyperaceae), known from pine-oak forest in Durango, Mexico, is described and illustrated. It belongs to Eleocharis subg. Eleocharis ser. Tenuissimae and can be distinguished by its diminutive size, with culms 2-13 mm long (including spikelets 1.3-1.7 mm long), and the folliform prolongations of its upper sheaths. It is the smallest species of Eleocharis thus far known.