Abstract: The number of software providers offering their applications as a Software-as-a-Service (SaaS) to exploit the benefits of cloud computing is increasing. New challenges to the cloud services discovery are imposed due to the SaaS services unique characteristics such as various and dynamic service offerings and the lack of standard description language. In this study, we propose OntSaaS, an ontology-based system for SaaS publication and discovery. Ont SaaS standardizes the advertisement process, serves as a semantic-based catalogue for the service offerings and provides competent search capabilities for the user. The main building blocks of the proposed system are the unified SaaS ontology and a semantic business-oriented matchmaking technique that maps requests and offers of cloud SaaS services. The proposed request-service matchmaking algorithm merges semantic-based services metadata with ontology-based hierarchical matching. Prototypical implementation and evaluation of the system proved its performance enhancement in respect of the service utility and success rate. Results showed that the concept recommendation approach employed decreased the service registration process time. Moreover, the proposed matchmaking algorithm similarity results revealed the actual relevance of the offered services to the user requests. Finally, the proposed ontology-based expansion approach for the user request improved the user opportunity to find appropriate services to his requirements in case of discovery partial match.