In terms of sustainable development, various forms of partnership are an important prerequisite for the development of companies, especially small and medium-sized enterprises (SMEs). Clusters are considered to be an effective form of support and innovation development for SMEs. Thus, enterprises working as a part of clusters are more efficient, flexible and resistant to external influences than large and hierarchical organizations. Clusters often arise spontaneously through the activities of local stakeholders who want to benefit from the synergy of the factors in the region, such as the existence of suppliers and customers, skilled labour force, specific resources and infrastructure. There are more than 20 clusters operating in the Slovak Republic. The majority of cluster members are SMEs. SMEs have many specificities and their engagement with clusters is influenced by various factors. For instance, many SMEs are often not sufficiently innovative or find it difficult to implement the results of their research and development into practical and successful innovation in the market. Bearing in mind the conditions of sustainable development, the authors examined the reasons why SMEs got engaged with clusters and enhanced their further development in terms of competitiveness. Moreover, the authors placed emphasis on their ability to innovate their products, technologies and/or services. Attention was also paid to the increased number of joint projects in the field of business, new market placement options and access to new information, as well as the acquisition of new partners, and employment growth in the regions in which these enterprises operate. The purpose of the paper was to identify the most relevant reasons why SMEs got engaged with clusters in terms of sustainable development. In order to achieve this purpose, an empirical study was carried out. The results were evaluated by quantitative tools of statistics (percentage, average values, standard deviation, chi-square test and Cramér’s V). The results showed that the most important reasons that made SMEs’ engage with clusters were entry to new markets and getting new partners.