IntroductionSince the WHO declared COVID-19 as a pandemic, the spread of the new coronavirus has been the focus of attention of scientists, governments and populations. One of the main concerns is the impact of this pandemic on health outcomes, mainly on mental health. Even though there are a few empirical studies on COVID-19 and mental health, so far, there is no systematic review about the impact of COVID-19 on mental health of young people and adults yet. We aim to critically synthesise the scientific evidence about the impact of the COVID-19 pandemic on the mental health of young people and adults.Methods and analysisA systematic review will be performed through eight databases: MEDLINE (Medical Literature Analysis and Retrieval System Online), ISI-of-Knowledge, CENTRAL (Cochrane Central Register of Controlled Trials), EMBASE (Excerpta Medica Database), SCOPUS, LILACS (Latin American and Caribbean Health Sciences Literature), PsycINFO (Psychology Information) and CNKI (Chinese National Knowledge Infrastructure), from inception until 30 June 2020. No restriction regarding the publication date, setting or languages will be considered. Preliminary search strategies were carried out on 29 March 2020 and will be updated in June 2020. The primary outcomes will be the prevalence and the severity of psychological symptoms in young people and adults (>18 years old) resulting from the impact of COVID-19 pandemic. Study selection will follow the Preferred Reporting Items for Systematic Reviews and Meta-Analyses checklist. Pooled standardised mean differences and 95% CIs will be calculated. The risk of bias of the observational studies will be assessed through the Methodological Index for Non-Randomised Studies (MINORS). Additionally, if sufficient data are available, a meta-analysis will be conducted. Heterogeneity between the studies will be determined by the I² statistics. Subgroup analyses will also be performed. Publication bias will be checked with funnel plots and Egger’s test. Heterogeneity will be explored by random-effects analysis.Ethics and disseminationEthical assessment was not required. Findings will be disseminated through peer-reviewed publication and will be presented at conferences related to this field.PROSPERO registration numberCRD42020177366.