The coronavirus disease 2019 (COVID-19) pandemic is a global health crisis with the potential to elicit and worsen psychiatric symptoms, particularly post-traumatic stress disorder (PTSD) symptoms. Identifying modifiable protective factors is critical for preventing and treating PTSD symptoms both during and following the COVID-19 pandemic. The present study examined associations of self-reported sleep quality and anticipatory threat appraisals of the pandemic with PTSD symptoms 6 months later in a sample enriched for pre-pandemic trauma exposure and PTSD. The sample included 590 adults (mean age 38.2 years) who completed a baseline survey in August/September 2020 and follow-up survey in March/April 2021. The sample was recruited from a pool of participants interested in a prior study about traumatic stress. Participants self-reported sleep quality and pandemic-related anticipatory threat appraisals at baseline. PTSD symptoms were assessed at baseline and follow-up. Baseline sleep quality was associated with PTSD symptoms at follow-up controlling for baseline PTSD symptoms (B = -2.49, p = 0.001). Perceived anticipatory threat of the pandemic moderated this association such that worse sleep quality was related to more severe PTSD symptoms at follow-up for participants with higher (B = -4.07, p < 0.001) but not lower (B = -0.43, p = 0.679) anticipatory threat about the COVID-19 pandemic. These findings suggest that poor sleep quality may enhance vulnerability to later PTSD symptoms during the pandemic, particularly among those individuals who perceived the pandemic as threatening for their future. Treatments that address sleep problems may be beneficial for reducing trauma-related symptoms during and following the global health crisis.