The genus Zygodon Hook. & Taylor is revised for southern South America (Andean-Patagonian and Fuegian regions). Thirteen taxa (twelve species and one variety) are recognized as occurring in the region: Z. bartramioides Malta, Z. chilensis Calabrese & F. Lara, Z. hookeri Hampe var. hookeri, Z. hookeri var. leptobolax (Müll. Hal.) Calabrese comb. nov., Z. intermedius Bruch & Schimp., Z. jaffuelii Thér., Z. magellanicus Dusén ex Malta, Z. menziesii (Schwägr.) Arnott, Z. papillatus Mont., Z. pentastichus (Mont.) Müll. Hal., Z. pichinchensis (Taylor) Mitt., Z. reinwardtii (Hornsch.) Bruch & Schimp. and Z. obtusifolius Hook. All taxa previously considered to grow in southern South America were studied, and many were typified. Because of the biogeographical connections with other Southern Hemisphere landmasses, some taxa from other areas (e.g., Australasia or South Africa) were also considered. The studied taxa are a good representation of the genus as a whole, thus allowing one to conclude that Zygodon, worldwide, consists of three subgenera: subg. Zygodon, the species of which have papillose leaf cells that are flat as seen in cross-section, and that can be further divided in two sections, sect. Zygodon defined by leaf basal cells not in differentiated bands and seta distally dextrorse, and sect. Stenomitrium Mitt. with leaf basal cells disposed in differentiated alternating bands and seta distally sinistrorse; subg. Codonoblepharon (Schwägr.) Calabrese stat. nov. comprising species with smooth, flat leaf cells in cross-section; and subg. Obtusifolium (Malta) Calabrese stat. nov., including species with papillose, bulging leaf cells. For each of the accepted taxa, keys, descriptions, illustrations, a complete list of synonyms previously used in the study area (sometimes expanded with names from abroad), ecology, distribution maps, variability and differentiation from related taxa are provided. Study of the peristome of Zygodon with the SEM provides new data about its structure and ornamentation.