Following its outbreak, the COVID-19 pandemic had strong negative effects on university students' stress and mental health worldwide. Using two longitudinal datasets from Germany (N = 504) and the U.S. (N = 893), we investigated how students' stress developed over the first two academic years during the pandemic. In both studies, we found elevated levels of students' stress at the beginning of the pandemic. In Germany, we found a significant intraindividual decrease in students' general stress experiences even before universities had returned to in-person classes. When examining specific stress facets in the U.S., we found that students' academic stress increased during the first pandemic year with remote teaching and decreased significantly after the university resumed normal operations, that is, in-person classes and on-campus residence. Students' practical stress decreased towards all later time points compared to the onset of the pandemic, whereas health stress continuously increased until the university resumed normal operations. We report differences by students' demographic backgrounds (gender, college generation status, childcare status, ethnicity, academic year) and discuss our findings against the background of the course of the pandemic in the particular context in which both studies were conducted.