Background: Improving sidewalks may encourage physical activity by providing safe, defined, and connected walking spaces. However, it is unknown if reduced health care expenditures assumed by increased physical activity offset the investment for sidewalk improvements. Methods: This cost-effectiveness analysis of sidewalk improvements in Houston, TX, was among adults enrolled in the Houston Travel-Related Activity in Neighborhoods Study, 2013–2017 . The 1-year change in physical activity was measured using self-report (n = 430) and accelerometry (n = 228) and expressed in metabolic equivalent (MET) hours per year (MET·h·y−1). Cost-effectiveness ratios were calculated by comparing annualized sidewalk improvement costs (per person) with 1-year changes in physical activity. Results: The estimated cost-effectiveness ratio were $0.01 and −$0.46 per MET·h·y−1 for self-reported and accelerometer-derived physical activity, respectively. The cost-effectiveness benchmark was $0.18 (95% confidence interval, $0.06–$0.43) per MET·h·y−1 gained based on the volume of physical activity necessary to avoid health care costs. Conclusions: Improving sidewalks was cost-effective based on self-reported physical activity, but not cost-effective based on accelerometry. Study findings suggest that improving sidewalks may not be a sufficient catalyst for changing total physical activity; however, other benefits of making sidewalks more walkable should be considered when deciding to invest in sidewalk improvements.