Small and medium-sized enterprises (SME) play a special role in the modern economy. At the same time, the difficulty in accessing sources of financing is the main barrier limiting the development of this sector. Despite this, studies of the capital structure of SMEs are performed less frequently than among large enterprises. The aim of this paper is to examine which capital structure theory best explains the financial decisions of SMEs in Poland. Additionally, an attempt is made to identify the main firm-specific capital structure determinants. The research material includes financial data from 2,820 SMEs in Poland operating in the 2011–2018 period. Static and dynamic panel models were applied to conduct the analysis. The study found that most SMEs in Poland behave in accordance with the pecking order theory. However, the results of testing the trade-off theory indicate that there may be a group of companies seeking an optimal capital structure according to the assumptions of this theory. The speed of adjustment is about 24%. The study confirmed a positive relationship between SME indebtedness and firm size. The same relationship was found for the growth rate. The opposite direction was identified for tangibility and liquidity.