Regular physical activity (PA) and reduced sedentary behavior (SB) are well-established factors in promoting health and lowering the risk of chronic diseases. However, their relationship with unplanned hospitalizations remains unclear. Our aim was to investigate the associations between PA and SB parameters and the risk of unplanned hospital admissions, and the length of hospital stays in older adults. We analyzed data from 657 older adults aged ≥ 66 years from the Swedish SNAC-K study (2016-2018). The ActivPAL3 accelerometer was used to assess PA and SB. Multi-adjusted Cox and Laplace regressions were applied to examine PA and SB in relation to 6-year unplanned hospitalizations. Negative Binomial Regression models were employed to examine their associations with the length of hospital stays. For each 1000 steps/day, 100 low-intensity PA (LPA) walking events, and 15 min/day of time spent in moderate-to-vigorous PA (MVPA), the risk of unplanned admissions decreased (hazard ratio [HR] 0.95, 95% confidence interval [CI] 0.91-0.99, HR 0.77, 95%CI 0.61-0.96, and HR 0.91, 95%CI 0.83-1.00, respectively). Higher step counts and MVPA were also associated with shorter hospital stays (IRR per 1,000 steps: 0.91, 95% CI: 0.86-0.96; IRR per 15 min MVPA: 0.85, 95% CI: 0.75-0.96). No significant associations were found for SB or walking events with hospital stay in fully adjusted models. Our results suggest that higher step counts and greater participation in MVPA are associated with a reduced risk of hospital admissions and shorter hospital stays in older adults. Additionally, a greater number of LPA walking events was linked to lower risk of hospital admissions. These results support the promotion of regular walking and frequent PA as strategies to reduce healthcare burden in aging populations.