Background: The Suicide Prevention Act was implemented in 2006 in Japan to promote various suicide prevention strategies. Aims: The present study examined the impact of the Suicide Prevention Act on recent suicide mortality rates in Japan. Method: Using an interrupted time-series design, we analyzed monthly mortality rates between January 1996 and December 2016. Death certificate data from vital statistics were obtained. Results: A total of 597,007 suicides (99.3% of all suicides) were analyzed. At the onset of the economic recession in 1998, a significant increase was observed in overall age-standardized mortality rates and sex-/age-specific populations, except for those aged 60 or older. The difference in trend between before and after implementation of the Suicide Prevention Act was not significant for overall or for any stratified populations. After the onset of the Tōhoku earthquake and tsunami of 2011, mortality rates declined for overall and for sex-/age-specific populations. Limitations: No information was available on what could have led to each suicide. Conclusion: The decline in mortality rates may be due to a significant and recent natural disaster. Further studies are needed to clarify plausible mechanisms for the decline in suicide rates following the Tōhoku disaster.