Background Increased systolic blood pressure variability (BPV) is associated with stroke, cardiovascular disease, and dementia and mild cognitive impairment. However, prior studies assessing the relationship between BPV and dementia or mild cognitive impairment had infrequent measurement of blood pressure or suboptimal blood pressure control. Methods and Results We performed a post hoc analysis of the SPRINT (Systolic Blood Pressure Intervention Trial) MIND (Memory and Cognition in Decreased Hypertension) trial. The primary outcome was probable dementia during follow‐up. We defined our exposure period, during which blood pressures were collected, as the first 600 days of the trial, and outcomes were ascertained during the subsequent follow‐up. BPV was measured as tertiles of systolic blood pressure standard deviation. We fit Cox proportional hazards models to our outcome. We included 8379 patients. The mean follow‐up was 3.2±1.4 years, during which 316 (3.8%) patients developed dementia. The mean number of blood pressure measurements was 7.8, and in the tertiles of BPV, the SD was 6.3±1.6, 10.3±1.1, and 16.3±3.6 mm Hg, respectively. The rate of dementia was 2.4%, 3.6%, and 5.4% by ascending tertile, respectively ( P <0.001). In the Cox models, compared with the lowest tertile of BPV, the highest tertile of BPV increased the risk of dementia in both unadjusted (hazard ratio [HR], 2.36; 95% CI, 1.77–3.15) and adjusted (HR, 1.69; 95% CI, 1.25–2.28) models. Conclusions In a post hoc analysis of the SPRINT MIND trial, we found that higher BPV was associated with the development of probable dementia despite excellent blood pressure control. Additional research is needed to understand how to reduce BPV and if its reduction lowers the risk of cognitive impairment and dementia.