Mexican Spanish (Spanish: español mexicano) is the variety of dialects and sociolects of the Spanish language spoken in Mexico and its bordering regions. Mexico has the world's largest number of Spanish speakers, more than double any other country. Spanish is spoken by over 99% of the population, being the mother tongue of 93.8%, and the second language of 5.4%.

The territory of contemporary Mexico is not coextensive with what might be termed Mexican Spanish, since linguistic boundaries rarely coincide with political ones. The Spanish spoken in the southernmost state of Chiapas, bordering Guatemala, resembles the variety of Central American Spanish spoken in that country, where voseo is used. Meanwhile, the Treaty of Guadalupe Hidalgo led to a large number of Mexicans residing in what had become US territory, and many of their descendants have continued to speak Spanish. In addition, the waves of 19th- and 20th-century migration from Mexico to the United States have greatly contributed to making Mexican Spanish the most widely spoken variety of Spanish in the United States. Finally, the Spanish spoken in coastal areas often exhibits certain phonetic traits in common with Caribbean Spanish rather than with that of central Mexico, and the Spanish of the Yucatán Peninsula is quite distinct from other varieties. It should also be noted that there is great variation in intonation patterns from region to region within Mexico. For instance, the Spanish of northern Mexico, including the traditional Spanish of New Mexico, is characterized by its own distinct set of intonation patterns.

Regarding the evolution of the Spanish spoken in Mexico, the Swedish linguist Bertil Malmberg points out that in Central Mexican Spanish—unlike most varieties in the other Spanish-speaking countries—the vowels lose strength, while consonants are fully pronounced. Malmberg attributes this to a Nahuatl substratum, as part of a broader cultural phenomenon that preserves aspects of indigenous culture through place names of Nahuatl origin, statues that commemorate Aztec rulers, etc. The Mexican linguist Juan M. Lope Blanch, however, finds similar weakening of vowels in regions of several other Spanish-speaking countries; he also finds no similarity between the vowel behavior of Nahuatl and that of Central Mexican Spanish; and thirdly, he finds Nahuatl syllable structure no more complex than that of Spanish. Furthermore, Nahuatl is not alone as a possible influence, as there are currently more than 90 native languages spoken in Mexico and Nahuatl never spread across the entire territory of modern Mexico.

Due to influence from indigenous languages, such as Nahuatl, Mexican Spanish has incorporated many words containing the sequences ⟨tz⟩ and ⟨tl⟩, corresponding to the voiceless alveolar affricate [t͡s] and the voiceless alveolar lateral affricate [t͡ɬ], present in many indigenous languages of Mexico, as in the words tlapalería [t͡ɬapaleˈɾia] ('hardware store') and coatzacoalquense [koat͡sakoalˈkense] ('from [the city of] Coatzacoalcos'). Mexican Spanish always pronounces the /t/ and /l/ in such a sequence in the same syllable, a trait shared with the Spanish of the rest of Latin America, that of the Canary Islands, and the northwest of the Iberian Peninsula, including Bilbao and Galicia. This includes words of Greek and Latin origin with ⟨tl⟩ such as Atlántico and atleta. In contrast, in most of Spain, the /t/ would form part of the previous syllable's coda, and be subject to weakening, as in [aðˈlantiko], [aðˈleta].

Some claim that in Mexican Spanish, the sequence /tl/ is really a single phoneme, the same as the lateral affricate of Nahuatl. On the other hand, José Ignacio Hualde and Patricio Carrasco argue that /tl/ is best analyzed as an onset cluster on the basis that Mexicans take the same amount of time to pronounce /tl/ as they do to pronounce /pl/ and /kl/. They predicted that if /tl/ were a single segment, it would have been pronounced quicker than the other clusters.

In addition to the usual voiceless fricatives of other American Spanish dialects (/f/, /s/, /x/), Mexican Spanish also has the palatal sibilant /ʃ/, mostly in words from indigenous languages—especially place names. The /ʃ/, represented orthographically as ⟨x⟩, is commonly found in words of Nahuatl or Mayan origin, such as Xola [ˈʃola] (a station in the Mexico City Metro). The spelling ⟨x⟩ can additionally represent the phoneme /x/ (also mostly in place names), as in México itself (/ˈmexiko/); or /s/, as in the place name Xochimilco—as well as the /ks/ sequence (in words of Greco-Latin origin, such as anexar /anekˈsar/), which is common to all varieties of Spanish. In many Nahuatl words in which ⟨x⟩ originally represented [ʃ], the pronunciation has changed to [x] (or [h])—e.g. Jalapa/Xalapa [xaˈlapa].

Regarding the pronunciation of the phoneme /x/, the articulation in most of Mexico is velar [x], as in caja [ˈkaxa] ('box'). However, in some (but not all) dialects of southern Mexico, the normal articulation is glottal [h] (as it is in most dialects of the Caribbean, the Pacific Coast, the Canary Islands, and most of Andalusia and Extremadura in Spain). Thus, in these dialects, México, Jalapa, and caja are respectively pronounced [ˈmehiko], [haˈlapa], and [ˈkaha].

In northwestern Mexico and rural Michoacan, [tʃ], represented by ⟨ch⟩, tends to be deaffricated to [ʃ], a phonetic feature also typical of southwestern Andalusian Spanish dialects.