BACKGROUND The study aim was to assess the prevalence of depression, anxiety, stress, and insomnia among resident doctors in Poland during the COVID-19 pandemic. MATERIAL AND METHODS The online anonymous survey was conducted among Polish resident doctors attending obligatory specialization courses organized by the Center of Postgraduate Medical Education between 2020 and 2021. The psychological impact of COVID-19 was measured using the Depression, Anxiety, and Stress Scale (DASS-21). The sleep problems were assessed using the Insomnia Severity Index (ISI). RESULTS Among 767 resident doctors participating in the study there were substantial levels of depression (14.7% mild, 26.2% moderate, 10.8% severe, and 9.5% extremely severe), anxiety (9.0% mild, 20.6% moderate, 8.3% severe, and 11.5% extremely severe), and stress (16.4% mild, 20.9% moderate, 15.7% severe, and 5.7% extremely severe), as well as substantial incidence of insomnia (58.0%), (38.3% subthreshold, 17.9% clinically moderate, and 1.7% clinically severe). Female doctors, physicians working directly with COVID-19 patients, and those who had COVID-19 themselves were at higher risk of depression, stress, and anxiety. Sleep disorders were more prevalent among doctors in surgical specializations, as well as those working directly with COVID-19 patients. CONCLUSIONS The COVID-19 pandemic in Poland appears to have negatively affected doctors' mental health. High levels of depression, anxiety, stress, and insomnia indicate that systemic solutions are needed. A spectrum of interventions should be explored to mitigate further strain on the physicians' psychological health in the post-pandemic work environment. It is necessary to focus on groups at particular risk, such as women, front-line doctors, doctors in health crisis, and residents in selected fields of medicine.