This book constitutes the thoroughly refereed post-conference proceedings of the 25th International Workshop on Algebraic Development Techniques, WADT 2020, held virtually in April 2020.The 7 revised papers presented together with an invited paper were carefully reviewed and selected from 9 submissions. The contributed presentations covered a range of topics about the algebraic approach to system specification, which encompasses many aspects of the formal design of software systems. Originally born as formal method for reasoning about abstract data types, the algebraic approach now covers new specification frameworks and programming paradigms (such as object-oriented, aspect-oriented, agent-oriented, logic, and higher-order functional programming) as well as a wide range of application areas (including information systems, concurrent, distributed, and mobile systems).