This study aimed to investigate the patterns of sick leave, as well as factors associated with sick leave due to COVID-19 during one year after the COVID-19 diagnosis, and sex-related aspects on sick leave. This nationwide study involved 11,902 individuals who received sickness benefits for COVID-19 during the first wave of the pandemic. Data from three Swedish registries were analyzed for sick leave that commenced between March 1 and August 31, 2020, with a follow-up period of 12 months. Sick leave due to COVID-19 was counted as the number of days with sickness benefits and required to include at least one registered COVID-19 diagnosis. The median duration of sick leave was 35 days, and 347 (2.9%) individuals continued their sick leave during the entire follow-up period. Furthermore, 1 year later, the cumulative incidence of sick leave was slightly higher in males (3.5%) compared to females (2.7%). Older age, being single with no children, diagnosed with the virus, medium income level, history of sick leave, and need for inpatient care were significantly associated with a higher duration of sick leave due to COVID-19, both in the total population and when stratified by sex. These results indicated that three out of 100 (3%) patients were still on sick leave 1 year after their COVID-19 diagnosis. Aspects regarding the importance of sick leave duration differed between males and females and comprised sociodemographic characteristics and need for inpatient care. The results indicated the complexity of sick leave due to COVID-19.