Over the past 10 years, cooperative banks in Poland have significantly increased their share and changed the structure of financial instruments in their portfolio. Asset structure is key to both performance and risk resilience. There can be various reasons for increasing the share of non-credit current assets, including regulatory requirements, the need for revenues diversification or effective liquidity management, and the effects of such decisions depend on various factors, whether macroeconomic, sectoral or individual. The paper aims to assess the role of securities for the stable and safe operation of cooperative banks in Poland and to try to answer the question of whether these relations are similar to those in the commercial banking sector. The theoretical part of the paper includes a short definition overview related to the studied problem, i.e. identification of concepts related to the safety and stability of banks, financial instruments and the specificity of cooperative banks. In addition, an analysis was made of the regularities that can be encountered when shaping the optimal structure of assets in banks. To verify the research hypothesis about the positive impact of financial instruments on the stability of cooperative banks, a linear regression model was used. For measuring the stability the Z-Score indicator was adopted. The research period covered annualised monthly data for the period 12.2010–09.2021. Obtained results confirm the importance of the role played by financial instruments in building cooperative banks’ stability. The increase in the level of financial instruments had a beneficial effect on bank stability, but only when it took place in a smooth manner and, in addition, when their share in the asset structure was small.