Emerging research demonstrates that sedentary behavior is associated with various individual chronic conditions, independent of physical activity. Despite multimorbidity (having at least 2 chronic diseases) being highly prevalent (≥ 23% of adults) and associating with premature mortality, limited research has examined the association between sedentary behavior and multimorbidity, which was the purpose of this research letter. Data from the 2005-2006 NHANES were used (analyzed in 2015). A multimorbidity index variable was created indicating the number of 14 morbidities each patient had; sedentary behavior and physical activity were assessed via accelerometry. For every 60 min/day increase in sedentary behavior, participants had a higher multimorbidity index (β(adjusted) = 0.08; 95% CI: 0.04-0.11, p<0.001) and had an 11% (OR(adjusted) = 1.11; 95% CI: 1.01-1.21; p = 0.03) increased odds of being multimorbid (i.e., having ≥ 2 morbidities). Sedentary behavior is associated with multimorbidity (independent of light-intensity physical activity and adherence to moderate-to-vigorous physical activity guidelines), which underscores the importance of minimizing prolonged sedentary behavior (in addition to promoting physical activity) among adults.