<p>The long-term relationship between posttraumatic growth (PTG) and wellbeing after the COVID-19 pandemic remains unclear. This study aimed to examine their levels and changes at the individual level over time as well as to explore the bidirectional associations in Japan, the United Kingdom (UK), and the United States (US). Four waves of online surveys were administered in July and October 2023 and January and April 2024 to residents aged 20–74 years. The wave 1 included 400 participants from Japan, 398 from the UK, and 393 from the US, of whom 308, 226, and 157, respectively, completed all four waves. A two-way multivariate analysis of covariance was conducted to analyze the differences in PTG and wellbeing by country and wave. The associations between PTG and wellbeing were analyzed using a random intercept cross-lagged panel model to assess the within-person associations. PTG and wellbeing were lower in Japan than in the UK and the US. PTG in the UK and the US declined over time. The random intercepts of PTG and wellbeing showed significant positive associations across all three countries, whereas no significant associations were generally observed between the within-person fluctuations of PTG and wellbeing. In all three countries, the associations between subjective economic status and the random intercepts of wellbeing were consistently strong. PTG and wellbeing are primarily associated at the between-person level, with little evidence of dynamic within-person effects. The causal or dynamic role of PTG in promoting emotional wellbeing is attenuated.</p>