Parks are an important setting for leisure-time physical activity. Understanding how to attract residents to parks and encourage park users to be physically active is an important public health initiative. Natural experiments are a research priority for investigating whether changes to the physical environment affect physical activity; however, natural experiments involving parks have rarely been conducted.This natural experiment examined whether improvements to a park in Victoria, Australia, increased its use and park-based physical activity of users.Observational data were collected on park use and park-based activity among park users at the intervention park and a control park at three time points; baseline (T1, August 2009); post-improvement (T2, March 2010); and 12 months after baseline (T3, August 2010). At each time point, observations were undertaken during three 1.5-hour periods each day on 9 days. Analyses were conducted in 2011.In the intervention park, there were significant increases from pre- to post-improvement in the number of park users (T1=235, T3=985) and the number of people observed walking (T1=155, T3=369) and being vigorously active (T1=38, T3=257). At the control park, counts of usage decreased over the same period and no differences in walking or vigorous activity were observed.Improving the features of a local neighborhood park may lead to increased usage and physical activity.