The COVID Stress Scales (CSS) were developed to measure stress in response to the COVID-19 pandemic. To further investigate the psychometric properties of the CSS, we used data collected in Poland across two waves of assessment (N = 556 at T1 and N = 264 at T2) to evaluate the factor structure, reliability (at the item and scale level), measurement invariance (across the Polish and Dutch translations of the CSS, and time), over time stability, and external associations of the Polish-language version of the CSS (CSS-PL). Overall, results suggest that the CSS-PL is psychometrically robust, largely invariant across the countries and time-lags considered. The CSS-PL was also positively related to other measures of COVID-19 fear, health anxiety, obsessive compulsive symptoms, anxiety, depression, and intent to receive a COVID-19 vaccine. This study thus provides considerable information about the CSS's items and scales, and lays the foundation for future investigations into COVID stress across time and different populations.