This book LNCS 15411 constitutes the refereed proceedings of the 19th International Conference and Workshops on Algorithms and Computation, WALCOM 2025, held in Chengdu, China, during February 28 - March 2, 2025. The 26 full papers were carefully reviewed and selected from 68 submissions. WALCOM 2025 covered a wide range of topics, including approximation algorithms, algorithmic graph theory and combinatorics, algorithmic and combinatorial aspects of logic, combinatorial algorithms, combinatorial optimization, combinatorial reconfiguration, computational biology, computational complexity, computational geometry, discrete geometry, data structures, experimental algorithm methodologies, graph algorithms, graph drawing, parallel and distributed algorithms, parameterized algorithms and complexity, network optimization, online algorithms, randomized algorithms, and string algorithms.