BACKGROUND: Physical activity confers considerable health benefits, but only half of U.S. adults report achieving levels of aerobic physical activity consistent with current guidelines. Walking is an excellent way for most people to increase their physical activity. The prevalence of walking among adults increased by 6 percentage points from 2005 to 2010, but it is unknown whether this increase has been sustained. PURPOSE: To evaluate trends in the prevalence of walking for transportation or leisure among U.S. adults between 2005 and 2015. METHODS: Nationally representative data from the 2005, 2010 and 2015 National Health Interview Survey (NHIS) cancer control supplements (n=78,741) were analyzed to estimate the age-adjusted prevalence of self-reported walking among adults ≥ 18 years. Walking was defined as engaging in at least one 10-minute bout of transportation or leisure walking in the past seven days. Estimates are reported for the total sample and stratified by sex. Linear and quadratic trends in walking prevalence from 2005 to 2015 were tested using logistic regression. RESULTS: The overall prevalence of self-reported walking increased significantly from 2005 to 2015, although a leveling off was observed between 2010 and 2015 (2005: 55.7%, 2010: 62.1%, 2015: 63.9%; p-value for linear and quadratic trends < 0.05). A similar trend was observed among men, with no significant difference in prevalence between 2010 and 2015 (2005: 54.3%, 2010: 61.8%, 2015: 62.8%; p-value for linear and quadratic trends < 0.05). Among women, the prevalence of walking demonstrated a significant linear increase from 2005 to 2015 with no significant quadratic trend; the increase in prevalence between 2010 and 2015 was significant (2005: 57.5%, 2010: 62.5%, 2015: 65.1%; p-value for linear trend only < 0.05). CONCLUSION: Overall, the proportion of U.S. adults who reported walking significantly increased from 2005 to 2015; however, the results suggest that this increase has slowed in recent years, especially among men. This finding highlights the importance of implementing effective programs and policies that promote walking and improve the walkability of communities, as described in Step it Up! The Surgeon General’s Call to Action to Promote Walking and Walkable Communities.