Access to water safe for consumption is critical for health and well-being, yet substantial structural barriers often necessitate household action to make water safer. Social norms about water treatment practices are understudied as a driver of personal water treatment practice. This study assesses reported and perceived water treatment practices among women in a rural, water insecure setting. We used cross-sectional data from a population-based study of women living with children under 5 years old across eight villages in southwest Uganda. Participants reported their typical household water treatment practices and what they perceived to be the common practices among most other women with young children in their own village. Modified multivariable Poisson regression models estimated the association between individual behavior and perceptions. Of 274 participants (78% response rate), 221 (81%) reported boiling water and 228 (83%) reported taking at least one action to make water safer. However, 135 (49%) misperceived most women with young children in their village not to boil their water, and 119 (43%) misperceived most to take no action. Participants who misperceived these norms were less likely to practice safe water treatment (e.g., for boiling water, adjusted relative risk = 0.80; 95% CI 0.69-0.92, P = 0.002), adjusting for other factors. Future research should assess whether making actual descriptive norms about local water treatment practices visible and salient (e.g., with messages such as "most women in this village boil their drinking water") corrects misperceived norms and increases safe water treatment practices by some and supports consistent safe practices by others.