Abstract Background Sickness absence (SA) due to mental disorders has strongly increased in Finland during recent years while sickness absence due to somatic diagnoses has continued to decrease. Research is lacking on recent trends in SA by occupational class, especially during years of increasing mental disorders. The aim of the study is to analyze these trends during 2011-2021. Methods National register data from the Social Insurance Institution of Finland (Kela), Statistics Finland and Finnish Centre for Pensions were linked for all employed Finns aged 25-64 for years 2011-2021 (yearly N about 2 million). SA was measured by sickness allowance, compensated by Kela after 10 days of absence. Age-standardized yearly prevalences of SA due to different diagnoses were calculated for women and men in four occupational classes over 2011-2021. Logistic regression was used to assess differences between occupational classes adjusted for age, marital status, education and region of residence. Results SA due to mental disorders has greatly increased in all occupational classes in Finland after year 2016 among both women and men, while SA due to musculoskeletal and other somatic diagnoses has continued to decrease. Increase in SA due to mental disorders has been largest among lower non-manual employees, especially among women, whereby also overall SA prevalence among female lower non-manual employees has reached the level of manual workers. Men show broadly similar trends but manual workers still have the highest overall SA prevalence. Relative adjusted differences between occupational classes decreased slightly between 2011 and 2021. Conclusions Clear occupational class differences persisted in diagnosis-specific SA throughout 2011-2021. However, the magnitude and order of the class differences changed along with increasing SA due to mental disorders. Occupational class differences should be taken into account when aiming to prevent further increases in SA due to mental disorders. Key messages • Sickness absence (SA) due to mental disorders has increased in all occupational classes in Finland after year 2016, while SA due to somatic diagnoses has continued to decrease. • Increase in SA due to mental disorders has changed the overall occupational class differences in SA in Finland largely because the increase has been strongest among lower non-manual employees.