During the last year, the COVID-19 outbreak put all the healthcare workers around the world at risk of physical and psychological sequelae. The general purpose of the present study was to assess the mental health of Italian healthcare workers during the COVID-19 outbreak and to identify high-risk groups. Here, we present results from the baseline assessment of the "Healthcare workers' wellbeing (Benessere Operatori)" project on a sample of 1055 healthcare workers. Participants completed the Depression Anxiety Stress Scale-21, the Insomnia Severity Index, the Impact of Event Scale-Revised, the State-Trait Anger Expression Inventory-2, and the Maslach Burnout Inventory. Healthcare workers who worked in COVID wards reported higher levels of anxiety, insomnia, post-traumatic stress, anger, and burnout, compared to those reported by the healthcare workers who worked in non-COVID wards. Moreover, nurses, both in COVID and non-COVID wards, were at higher risk of experiencing psychological distress compared to other groups of healthcare workers. These findings highlight the importance of implementing targeted psychological interventions for healthcare workers operating in COVID wards and nurses, who seem to be the most vulnerable categories.