A taxonomic revision of the genus Amaranthus (Amaranthaceae) in Italy is here presented. Field surveys were carried out during the period 2006–2014. 58 herbaria (both European and American, including 12 personal herbaria) were consulted (more than 3,000 specimens were examined) as well as extensive literature was analized. Twenty-seven non-hybrid taxa (twenty-four species, and six varieties) are recognized (A. crassipes and A. graecizans subsp. graecizans are considered doubtful for the flora of Italy). Three taxa (A. blitum, A. cacciatoi, and A. graecizans subsp. sylvestris) are native, one (A. bouchonii) has doubtful origin, while the others are to be considered aliens, mostly neophytes native to the Americas. Information about nomenclature (accepted names, main synonyms, and types), morphology, chromosome number, chorology (for native taxa) or alien status (for exotic taxa, at national and regional levels), occurrence in Italy (at regional scale), ecology (preferential habitat, phenology, elevation), taxonomic annotations are provided for each taxon, as well as original photos were prepared. Diagnostic keys at species and infraspecific levels (for A. blitum subsp. blitum s.l., A. emarginatus s.l., and A. graecizans s.l.) are given. An isolectotype for the name A. bouchonii was found at Z. A list of the seven hybrids recorded and their main morphological characteristics are also given. Among them, A. × mauritii is recorded in the present study for the first time in Italy. The nomenclatural change Amaranthus × pyxidatus comb. et stat. nov. is proposed. Two specimens preserved at MPU are designated as lectotype and isolectotype of the name A. × mauritii s.s., while for A. × mauritii f. ramosissima the holotype was found; the two names are to be considered heterotypic synonyms (new synonymy).