Confinement by COVID-19 was a stressful period that could potentially trigger awake bruxism (AB) and/or sleep bruxism (SB) behaviors. This study aims to characterize the AB and SB behaviors reported by Portuguese dental students before the pandemic and during the first period of mandatory confinement by COVID-19. Dental students were included in this longitudinal study. They answered the Portuguese validated version of the Oral Behavior Checklist (OBC) before the COVID-19 pandemic emerged (T1) and one month after mandatory confinement started in Portugal (T2). Descriptive statistics and the linear-by-linear association test were performed to assess changes over time (p ≤ 0.05). Sixty-four dental students (mean age 22.5 ± 2.8 years; 81.5% females) completed the study protocol. Considering AB, there was a general increase of the behavior from T1 to T2. The percentage of participants who reported to “grind their teeth when waking up” just few times decreased (p < 0.001) and the percentage of participants who reported “feeling discomfort/tension in the facial muscles when waking up just few times” increased (p = 0.019). Considering SB, there was a significant decrease of the behavior in all samples. The number of “None” report to grinding teeth during sleep or when waking up increased (p = 0.012). An increase in the self-reporting of feeling discomfort in masticatory muscles when awake or sleeping was observed (p = 0.028). The percentage of participants who did “not remember” any AB or SB activity decreased (p < 0.050). The confinement due to COVID-19 resulted in a forced change in dental students’ lifestyles that resulted in an increase of reported AB and a decrease of reported SB. Clinical Significance: In case of confinement periods, students should be encouraged to try normalizing their daily life by creating healthy routines and, by doing so, reducing the possible predisposition to bruxism and its consequences.