The microservices architectural style is gaining more and more momentum for the development of applications as suites of small, autonomous, and conversational services, which are then easy to understand, deploy and scale. One of today’s problems is finding the adequate granularity and cohesiveness of microservices, both when starting a new project and when thinking of transforming, evolving and scaling existing applications. To cope with these problems, the paper proposes a solution based on the semantic similarity of foreseen/available functionality described through OpenAPI specifications. By leveraging a reference vocabulary, our approach identifies potential candidate microservices, as fine-grained groups of cohesive operations (and associated resources). We compared our approach against a state-of-the-art tool, sampled microservices-based applications and decomposed a large dataset of Web APIs. Results show that our approach is able to find suitable decompositions in some 80% of the cases, while providing early insights about the right granularity and cohesiveness of obtained microservices.