Background: Sedentary behavior is associated with adverse health effects. Insights into associated determinants are essential to prevent sedentary behavior and limit health risks. Sedentary behavior should be viewed as a distinct health behavior; therefore, its determinants should be independently identified. Purpose: This study examines the prospective associations between a wide range of midlife determinants and objectively measured sedentary time in old age. Methods: Data from 565 participants (age 73-92 yr) of the AGESII-Reykjavik Study were used. Participants wore an accelerometer (ActiGraph GT3X) on the right hip for seven consecutive days. On average, 31 yr earlier (during midlife), demographic, socioeconomic, lifestyle, and biomedical factors were collected. Linear regression models were used to examine prospective associations between midlife determinants and sedentary time (<100 counts per minute) in old age. Results: After adjustment for sex, age, follow-up time, minutes of moderate to vigorous physical activity, body mass index, health status, mobility limitation, and joint pain in old age, the midlife determinants not being married, primary education, living in a duplex or living in an apartment (vs villa), being obese, and having a heart disease were associated with, on average, 15.3, 12.4, 13.5, 13.3, 21.8, and 38.9 sedentary minutes more per day in old age, respectively. Conclusions: This study shows that demographic, socioeconomic, and biomedical determinants in midlife were associated with considerably more sedentary time per day in old age. These results can indicate the possibility of predicting sedentariness in old age, which could be used to identify target groups for prevention programs reducing sedentary time in older adults.