This book constitutes therefereed proceedings of the 12th Latin American Symposium on TheoreticalInformatics, LATIN 2016, held in Ensenada, Mexico, in April 2016. The 52 papers presentedtogether with 5 abstracts were carefully reviewed and selected from 131submissions. The papers address a variety of topics in theoretical computerscience with a certain focus on algorithms (approximation, online,randomized, algorithmic game theory, etc.), analytic combinatorics and analysisof algorithms, automata theory and formal languages, coding theory and datacompression, combinatorial algorithms, combinatorial optimization,combinatorics and graph theory, complexity theory, computational algebra,computational biology, computational geometry, computational number theory,cryptology, databases and information retrieval, data structures, formalmethods and security, Internet and the web, parallel and distributed computing,pattern matching, programming language theory, and random structures.