We investigated the cross-sectional and prospective associations between different sedentary behaviors and cognitive function in a large sample of adults with data stored in the UK Biobank. Baseline data were available for 502,643 participants (2006-2010, United Kingdom). Cognitive tests included prospective memory (baseline only: n = 171,585), visual-spatial memory (round 1: n = 483,832; round 2: n = 482,762), fluid intelligence (n = 165,492), and short-term numeric memory (n = 50,370). After a mean period of 5.3 years, participants (numbering from 12,091 to 114,373, depending on the test) also provided follow-up cognitive data. Sedentary behaviors (television viewing, driving, and nonoccupational computer-use time) were measured at baseline. At baseline, both television viewing and driving time were inversely associated with cognitive function across all outcomes (e.g., for each additional hour spent watching television, the total number of correct answers in the fluid intelligence test was 0.15 (99% confidence interval: 0.14, 0.16) lower. Computer-use time was positively associated with cognitive function across all outcomes. Both television viewing and driving time at baseline were positively associated with the odds of having cognitive decline at follow-up across most outcomes. Conversely, computer-use time at baseline was inversely associated with the odds of having cognitive decline at follow-up across most outcomes. This study supports health policies designed to reduce television viewing and driving in adults.