Background: According to the WHO, the COVID-19 pandemic could have a negative impact on the mental health of individuals, such as an exacerbation of existing difficulties. Individuals with ADHD may be specifically challenged by the pandemic. Aims: To provide a systematic review of evidence regarding the COVID-19’s impact on mental health of individuals with ADHD during the COVID-19 lockdown. Methods: This registered review (PROSPERO ID CRD42021238770) adhered to Prisma guidelines. Systematic searches in electronic databases PubMed and PsycINFO were carried out. A total of 12 studies covering 3,028 subjects were included. Results: COVID-19 pandemic is associated with increased ADHD symptoms and psychological difficulties. Some studies reported that individuals experienced positive outcomes. The methodological quality of the studies was low to moderate. Conclusion: COVID-19 can affect the mental health of individuals with ADHD negatively, although methodological limitations should be considered. Further research should generate knowledge about long-term effects impact of the pandemic and about appropriate support.