Background In 2011, New York City (NYC) parks and beaches became smoke-free. There is currently little research evaluating the impact of such laws on smoking behaviour at the population level.

Methods We used an interrupted time-series study design to analyse data from the New York State Adult Tobacco Survey to assess the law's impact using the rest of New York State as a comparison. Trends in how frequently respondents noticed people smoking in parks and beaches were analysed between the third quarter of 2009 and the fourth quarter of 2012, comparing NYC to the rest of the state.

Results The trend in the frequency of NYC residents noticing people smoking in local parks and beaches decreased significantly over the six quarters after the law took effect. There was no comparable decline among residents in the rest of the state. An increase in the number of respondents who never noticed people smoking in NYC contributed to this decline.

Conclusions These results are consistent with previous studies and provide population-level evidence that suggest the law has reduced smoking in parks and on beaches.