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ió is a suffix form used in Spanish verb conjugation, specifically the third-person singular preterite ending for regular -er and -ir verbs. It is written with an acute accent on the i (í) to indicate stress on the syllable, distinguishing it from other tense endings.

In standard verbs, -ió marks the completed past action for él, ella, or Ud. examples include comer -> comió (he/she/you ate) and vivir -> vivió (he/she/you lived). The same ending appears in many other -er and -ir verbs in the preterite, such as beber -> bebió and escribir -> escribió (note that escribir is irregular in stem but still uses -ió in the form of its él/ella/Ud. preterite).

Some verbs with stems ending in a vowel produce a spelling shift in this form, yielding a -yó ending instead of a plain -ió. Common examples include caer -> cayó, creer -> creyó, leer -> leyó, oír -> oyó, incluir -> incluyó. These forms retain the same general function of the preterite but reflect phonological changes to maintain pronunciation.

The ending -ió does not apply to -ar verbs in the preterite; those use -ó for third-person singular (él/ella/Ud. form). Overall, -ió is a key marker of past, completed actions in many Spanish verbs and interacts with irregular spellings in a number of well-known verbs. See also Spanish verb conjugation, preterite tense, and irregular verbs.