Objectives: The aim of the research is to examine the effect of the pulmonary rehabilitation program on the strength of the respiratory muscles of patients with post-COVID syndrome. Study design: The research was conducted as a cross-sectional study. Subjects and methods: Data for the research were collected from the Hospital Information System on patients who participated in the pulmonary rehabilitation program at the University Hospital Centre Zagreb, Clinic for Lung Diseases Jordanovac, between January 2021 and December 2022, in terms of gender, age, the time elapsed from the disease to the start of rehabilitation, respiratory muscle strength (PImax and PEmax), functional capacity (6MWT), pulmonary function tests including spirometry (FVC, FEV1, FEV1/FVC), and diffusion capacity of lungs. All patients participated in the program after more than two months of illness. Results: This study included 80 patients (37 men, 43 women) with a mean age of 52 years who met the inclusion criteria. The effects of pulmonary rehabilitation on patients' respiratory muscle strength showed statistically significant differences (P < 0.001), with effect sizes ranging from small to large (Cohen's d from 0.39 to 1.07). The effect and difference are very large for PImax expressed as a percentage (Cohen’s d = 0.99 with an arithmetic mean difference of 18.16). In addition, the rehabilitation program significantly improved the patients' parameters of the 6-minute walk test. The measured pulmonary function parameters (FVC, FEV1, DLCO) show statistically significant differences (P < 0.05) except for the Tiffeneau index. However, the effect size of pulmonary rehabilitation is very small to small (Cohen's d = 0.18 to 0.48). Conclusion: Based on the conducted research, it is concluded that the pulmonary rehabilitation program has a statistically and clinically significant positive effect on respiratory muscle strength in patients with the post-COVID syndrome.