Mateo is a Spanish form of the name Matthew. The forms Matias, Matheo, Matejo, and Teo are also used, but Mateo is by far the most common form. It is used in Mexico, most of Central America, much of South America, and in Spain. It is a very common name, although not as common as Matthew is in English speaking counties.
Mateo is pronounced either as mah-TEH-oh or mah-TAY-oh. It originates, as does Matthew, from the Hebrew name Matityahu, meaning 'Devoted to God'. While it is traditionally a given name, it has also come to be used as a surname, and San Mateo ('Saint Matthew') is a common place name.
Mateo is also used in Esperanto and Venetian, while the form Matteo is used in Italian. Catalan uses the form Mateu. It has not yet entered English on any noteworthy scale, but I continue to hold out hope, as it is one of my favorite names.