Since the outbreak of the COVID-19 pandemic, outpatient nurses have been exposed to a double burden of already known occupational and new pandemic-related stressors. Recent studies suggest that increased pandemic-related stress can affect mental health and promote the development of negative mental health outcomes for nurses. This includes a decrease in sleep quality and work engagement. In addition, certain groups appear to be particularly vulnerable to pandemic-related stress. The aim of this study was to investigate the stress perception of German outpatient nurses during the COVID-19 pandemic. The aim was to determine associations between their pandemic-related stress and variables such as sleep quality, work engagement, pandemic-related worries and concerns. For this purpose, a questionnaire was developed based on well-established measurement instruments such as the 10-item Perceived Stress Scale, the Pittsburgh Sleep Quality Index and the Copenhagen Psychosocial Questionnaire to conduct a cross-sectional online survey among outpatient nurses from Germany. Participants (n = 166) showed rather moderate overall pandemic-related stress levels, good sleep quality, high work engagement, and moderate pandemic-related worries and concerns. Pandemic-related stress proved to be a predictor of decreased sleep quality and work engagement of outpatient nurses with weak effect sizes. Despite the surprisingly moderate stress levels, the effects of pandemic-related stress on selected aspects of participants’ mental health could be demonstrated. Therefore, behavioural and organisational health promotion measures are recommended to support outpatient nurses during the pandemic. However, further research is needed to determine the causal relationships and long-term effects of pandemic-related stress on the mental health of outpatient caregivers.