: The current COVID-19 pandemic is associated with a variety of stressors. Preliminary research has demonstrated that general public are experiencing a range of psychological problems, including stress-related disturbances. However, to date, there is not much research on the prevalence of adjustment disorder during the current pandemic. : This study aimed to assess the prevalence and severity of symptoms of adjustment disorder compared to posttraumatic symptoms, depression and generalized anxiety in a large sample of adult Poles, in the first phase of the current pandemic. : Self-report data from a web-based sample ( = 1,742) was collected between March 25 and April 27, just after the introduction of nationwide quarantine measures in Poland. : The current COVID-19 pandemic was a highly stressful event for 75% of participants and the strongest predictor of adjustment disorder. Increased symptoms of adjustment disorder were reported by 49%, and they were associated with female gender and not having a full-time job. However, after exclusion of co-occurring symptomatology, 14% of the sample were finally qualified as meeting diagnostic criteria of adjustment disorder. A substantial proportion of the sample screened also positive for generalized anxiety (44%) and depression (26%); the rate for presumptive PTSD diagnosis was 2.4%. : High rates of negative mental health outcomes were found in the Polish population in the first weeks into the COVID-19 pandemic and lockdown measures. They indicate the intense current stress-related symptoms in the early phase of the pandemic and warrant further monitoring on population's mental health.