Ceceo is a phenomenon in the Spanish language whereby the voiceless interdental fricative (International Phonetic Alphabet , the "th" in think) is used in place of the voiceless dental fricative /s/. In that respect, it is not unlike a lisp.

In standard Castilian Spanish, the letter s is always pronounced /s/, whereas the letter c (before the vowels e and i) and the letter z are pronounced /θ/. In all other dialects of Spanish this distinction has been long lost and /θ/ does not exist, being replaced by /s/ in all cases.

For speakers who exhibit ceceo, however, /s/ is replaced by /θ/.

For example, la casa ("the house") is pronounced as /la 'kasa/ in all dialects of Spanish, whereas la caza ("the hunt") is pronounced as /la 'kaθa/ in Castilian Spanish and /la'kasa/ in all other dialects. But a person with ceceo pronounces both of these /la 'kaθa/.

Ceceo most commonly occurs as a dialectal feature in Andalusian Spanish, but the term is also used to refer to a lisp in other dialects. Rather ironically, ceceo is regularly pronounced /se'seo/ in Latin American Spanish.