INTRODUCTION: This work presents an educational board game based on graph grammars to develop computational thinking skills. Graph grammars are formal languages that generalizes Chomsky’s grammars, where strings are replaced by graphs. OBJECTIVE: This game aims to explore the relations between graph grammars and computational thinking skills. It provides an alternative approach to promote computational thinking while introducing formal methods to K-12. METHOD: The game was developed under the ENgAGED process, a computer science educational games development methodology that meets design and instructional theories. The process covers analytical, conceptual, construction, application and evaluation phases. RESULTS: Four groups of students were evaluated with graph grammar tests. Two groups of students had positive results, increasing their average performances on the tests by 28% and 3%. One group kept its performance and the other group was not considered due to its lack of cooperation. CONCLUSION: This work provides “The Last Tree” as product, an instructional tool wherein the students manage a graph grammar while developing computational thinking skills. The game highlights graph grammars features such as: being visual, intuitive and yet formal languages; and high stimulation of data collection and analysis. The application and the tests showed the link between playing performance and general graph analysis and management abilities.