In Spanish grammar, voseo (Spanish pronunciation: [boˈse.o]) is the use of vos as a second-person singular pronoun, along with its associated verbal forms, in certain regions where the language is spoken. In those regions it replaces tuteo, i.e. the use of the pronoun tú and its verbal forms. Voseo can also be found in the context of using verb conjugations for vos with tú as the subject pronoun (verbal voseo), as in the case of Chilean Spanish, where this form coexists with the ordinary form of voseo.

In all regions with voseo, the corresponding unstressed object pronoun is te and the corresponding possessive is tu / tuyo.

Vos is used extensively as the second-person singular in Rioplatense Spanish (Argentina and Uruguay), Eastern Bolivia, Paraguayan Spanish, and Central American Spanish (El Salvador, Guatemala, Honduras, Nicaragua, Costa Rica, southern parts of Chiapas and some parts of Oaxaca in Mexico). Vos had been traditionally used even in formal writing in Argentina, El Salvador, Costa Rica, Nicaragua, Paraguay, the Philippines and Uruguay. In the dialect of Argentina, Paraguay and Uruguay (known as 'Rioplatense Spanish'), vos is also the standard form of use, even in mainstream media. In Argentina, particularly since the last years of the 20th century, it is very common to see billboards and other advertising media using voseo.

Vos is present in other countries as a regionalism, for instance in the Maracucho Spanish of Zulia State, Venezuela (see Venezuelan Spanish), in the Azuero peninsula of Panama, in various departments in Colombia, and in parts of Ecuador (Sierra down to Esmeraldas). In Peru, voseo is present in some Andean regions and Cajamarca, but the younger generations have ceased to use it. It is also present in Ladino (spoken by Sephardic Jews throughout Israel, Turkey, the Balkans, Morocco, Latin America and the United States), where it replaces usted. In the United States, Salvadoran Americans are by far the largest voseo users followed by other Central Americans, including Guatemalans, Hondurans, Nicaraguans, and Costa Ricans.

Voseo is seldom taught to students of Spanish as a second language, and its precise usage varies across different regions. Nevertheless, in recent years it has become more accepted across the Spanish-speaking world as a valid part of regional dialects.