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Background: The COVID-19 pandemic has had an impact on mental health and alcohol use in the US, however there is little research on its impacts on cannabis use. Considering the role of cannabis as a coping strategy or self-medicating behavior, there is a need to understand how individuals who use cannabis have adapted their use amid the pandemic. Therefore, this study examined changes in self-reported cannabis use among US adults in the context of COVID-19 pandemic by (1) describing trends of use during the first 8 months of the pandemic among adults who used cannabis in this period; and (2) characterizing trajectories of use within sociodemographic subgroups and by state cannabis policy status. Methods and Findings: The sample consisted of 1,761 US adults who used cannabis at least once during the 8 month study period from the nationally representative Understanding America Study. Linear mixed-effect models were used to model changes in the number of days of past-week cannabis use across 16 waves from March 10, 2020, to November 11, 2020. Compared to early March, the number of days cannabis was used per week was significantly higher at the start of April (β=0.11, 95% CI=0.03, 0.18) and May (β=0.21,95% CI=0.05, 0.36). In subsequent months (June - November), the number of days of cannabis use returned to levels comparable to early March. Trajectories of cannabis use across the study period generally did not differ across sociodemographic groups. Conclusions: In the US, adults who used cannabis reported statistically significant increases in cannabis use days at the start of the pandemic (April - May 2020), compared to March 2020, in the overall sample and among several sociodemographic groups. In Summer and Fall 2020, cannabis use days attenuated to levels comparable to March. Though increases in use were marginal among many groups, the evolving pandemic and the growing concern for the mental health of segments of the U.S. population warrant close monitoring of coping behaviors, including substance use.