Objective: Compared to other age groups, young adults are at risk of suffering COVID-19 pandemic-related psychological problems. Prior research suggests that such adverse events (like the pandemic) can initiate adaptive psychological changes, referred to as posttraumatic growth (PTG); however, limited studies have examined the moderating role of PTG among young adults in the United States with regard to COVID-19-related distress. Thus, this study examines whether pandemic-related distress has a differential effect on young adults’ mental health symptoms based on varying levels of PTG. Method: Using the data from 661 participants who completed all three waves of the CARES study (T1: April–August 2020; T2: September 2020–March 2021; T3: April–May 2021), a moderator analysis was performed using the PROCESS Macro to determine the moderating effect of PTG. Results: In our data, higher levels of PTG buffered the effects of pandemic-related distress from 2020 on depressive symptoms in 2021; PTG did not show a moderating effect with anxiety as an outcome. Conclusions: Understanding the moderating role of PTG would help to further understand the mental health trajectories of young adults in the United States who are particularly distressed by the pandemic. Our findings suggest the importance of further exploring contributors to PTG for young adults, particularly among those who have experienced high levels of pandemic-related distress to date.