The grades of students have been shown to severely decrease during the transition from high school to university. Although past findings have shown that procrastination negatively relates to academic achievement, no study has examined the influence of procrastination on the grade trajectory of students during this critical academic transition. Therefore, our main goal was to use a prospective design to examine the potentially detrimental role of procrastination on the grade trajectory from high school to the first year at university. We also examined the influence of procrastination over and above five demographic and socio-economic control variables. A sample of 269 first-year university students completed a procrastination questionnaire during their first semester at university. The official grades at three time points were obtained via the Registrar's Office: (1) the average grade of the last year of high school; (2) the average grade of the first semester at university; and (3) the average grade of the second semester at university. Results of piecewise multilevel growth modeling revealed that higher procrastination was associated with a larger grade decrease from high school to the first semester at university, which was maintained during the second semester at university even after considering demographic and socio-economic control variables. This study underscores that procrastination is a risk factor that tends to exacerbate the achievement difficulties experienced by students during the transition from high school to university.