This cross-sectional observational study aims to estimate the number of days absent from work due to health-related problems among employed individuals with multimorbidity and to quantify the lost productivity value from these absences. Data were obtained from the Belgian Health Interview Survey 2018, comprising employed individuals aged 15-64 (N = 4096). We examined 12 chronic conditions and 57 dyads. The Human Capital Approach was used by multiplying the reported number of days absent by the average wage per person per day, utilizing stratified gross wages from the Belgian Statistical Office. Approximately one-third of the study population reported multimorbidity. For individuals with zero to four+ chronic conditions, mean days of absence were 5.5 (95% CI: 2.3-8.8), 6.8 (95% CI: 2.9-10.7), 14.8 (95% CI: 10-19.6), 24 (95% CI: 17.8-30.2), and 36.2 (95% CI: 30.4-42), respectively. Depression (€3089; 95% CI: 2129-4049), diabetes (€2315; 95% CI: 962-3668), arthropathies (€1972; 95% CI: 1101-2844), and cancer (€1848; 95% CI: 598-3099), as standalone conditions, were associated with the greatest productivity losses. The effects were amplified up to seven times with the co-occurrence of multiple chronic conditions. We estimated 34.2 million days absent or €7.5 billion in lost productivity due to health-related work absenteeism among working-age employed individuals with multimorbidity in 2018. At the population level, the coexistence of two musculoskeletal disorders was linked to the highest aggregated productivity loss. At the individual level, the coexistence of a mental health condition and a somatic condition was associated with the highest average productivity loss per capita. The indirect cost due to health-related absence from work for individuals with multimorbidity in Belgium is high, and in many cases, exceeds the direct cost of treatment.