Background: Posttraumatic stress symptoms (PTSSs) and alcohol, tobacco, or nicotine use are frequently associated conditions. The COVID-19 pandemic has been a stressful situation globally and has worsened mental health conditions and addictions in the population. Our systematic review explores the links between PTSSs and (1) alcohol use and (2) tobacco or nicotine use during the COVID-19 pandemic. Methods: We searched the PubMed, PsycINFO, and Web of Science databases for studies published between January 2020 and 16 December 2021. We included studies published in English concerning adults or adolescents. Included articles dealt simultaneously with the COVID-19 pandemic, PTSSs, and alcohol, tobacco, or nicotine use. The reports included were cross-sectional, longitudinal, or cohort studies. We categorized the reports according to the population explored. Our main outcomes are the impacts of PTSSs on (1) alcohol use and (2) tobacco and nicotine use and their relation to COVID-19-related stressors (worries, exposure, lockdown, and infection, either of self or relatives). Results: Of the 503 reports identified, 44 were assessed for eligibility, and 16 were included in our review, encompassing 34,408 participants. The populations explored were the general population, healthcare workers, war veterans, patients with substance use disorders, and other vulnerable populations. Most studies were online surveys (14) with cross-sectional designs (11). Every study explored alcohol use, while only two assessed tobacco use. In most populations explored, a high level of PTSSs was associated with alcohol use increase. COVID-19-related stress was frequently correlated with either high PTSSs or alcohol use. In healthcare workers, PTSSs and alcohol use were not associated, while COVID-19 worries were related to both PTSSs and alcohol use. Discussion: 1. PTSSs and increased alcohol use are frequently associated, while COVID-19 worries might trigger both conditions and worsen their association. Alcohol use increase may represent either an inadequate way of coping with PTSSs or a vulnerability amid the COVID-19 pandemic, leading to PTSSs. As most studies were cross-sectional online surveys, longitudinal prospective studies are needed to ascertain the direction of the associations between these conditions. These studies need to be sufficiently powered and control for potential bias and confounders. 2. Our review highlighted that research about PTSSs and tobacco or nicotine use is scarce.