In the Web of Things community, the SSN/SOSA ontology, a W3C recommendation, has established itself as a foundational framework for representing semantic sensor networks, modeling sensors, observations, and actuations. However, an explicit representation of message protocols is missing, which limits semantic interoperability in machine-to-machine communication scenarios. To close this gap, we propose MQTT4SSN as an extension to SSN/SOSA that semantically models the MQTT protocol. Furthermore, the ontology addresses use cases without SSN/SOSA implementations by exclusively describing the MQTT messaging protocol. MQTT4SSN represents MQTT entities such as brokers, clients, control packets, topics, and payload metadata, linking them to SSN/SOSA concepts to enable end-to-end traceability between sensing semantics and communication semantics. The ontology captures heterogeneous payload formats, encodings, and transport metadata, enabling machine-interpretable description and integration of transmitted content. MQTT4SSN represents an extensible and reusable resource that is accessible according to the FAIR principles and documented as an Ontology Specification Draft. Ontology: https://doernern.github.io/MQTT4SSNOntology/MQTT4SSN.owl Documentation: https://doernern.github.io/MQTT4SSNOntology/documentation/index-en.html WebVOWL: https://doernern.github.io/MQTT4SSNOntology/documentation/webvowl/index.html OOPS!: https://doernern.github.io/MQTT4SSNOntology/documentation/OOPSevaluation/oopsEval.html GitHub: https://github.com/doernern/MQTT4SSNOntology License: CC BY-NC-SA 4.0 DOI: 10.5281/zenodo.16704302

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