Background: Few studies have examined the effect of pandemics on suicide-related outcomes. Aims: We examined whether suicidal ideation levels among the general population changed owing to the COVID-19 pandemic by tracking individuals between January and April 2020. Method: We used a prospective observational longitudinal design (n = 6,683) to conduct online surveys of the general adult population in Japan before (baseline) and during the pandemic (follow-up). Results: Suicidal ideation levels were significantly lower during than before the pandemic; however, the effect size was small (r = .07). Participants who were younger, with unstable employment, without children, with low income, and receiving psychiatric care were more likely to have higher suicidal ideation levels during the pandemic. Limitations: The dropout rate may have affected the results. COVID-19 cases and deaths in Japan were relatively lower than in other developed countries. Conclusion: Although the short-term impact of COVID-19 on suicidal ideation is limited, relatively young and economically vulnerable individuals are more likely to show exacerbated suicidal ideation during the pandemic.