Objectives: Construct and verify the psychometric properties of a questionnaire for assessing the level of physical activity of elderly people in institutional care. Further investigate the relationship between physical activity and division by gender, age categories, body mass index (BMI), limiting factors and comorbidities. Respondents and Methods: 200 beneficiaries of institutional care in nursing homes in the city of Zagreb took part in the study after several months of anti-epidemic measures which were in place during the COVID-19 pandemic. Beneficiaries 65 years of age and older were included who according to their functional ability and state of health receive the 1st level of accommodation services in the social care system. An integrative approach was used in assessing the level of physical activity by combining a self-assessment model with a 7-day motor monitoring. Results: The Physical activity-Dolor Dyspnoea Fatigo questionnaire (PA-DDF) showed appropriate relevance and comprehensibility according to the results of the content validity index (CVI > 0.80). Cronbach alpha coefficient values indicate good reliability of the physical activity domain (α = 0.881) and excellent reliability of the limitation domain (α = 0.949). The one-factor structure of the physical activity domain and the three-factor structure of the limitation domain were confirmed. The physical activity domain of the PA-DDF questionnaire positively correlates with the result obtained on the Physical Activity Scale for the Elderly questionnaire, the average number of steps taken and medium-intensity activities through the diary and netHR values ranging from 0.724 to 0.699. This domain is negatively correlated with low-intensity activities and sedentary behavior monitored through diary and netHR values ranging from -0.601 to 0.280. In the limitation domain, fatigue achieves a dominant negative implication with the physical activity domain ranging from -0.674 to -0.629. A significantly higher level of physical activity is achieved by men, younger people and beneficiaries. Although beneficiaries with minor limitations achieve a significantly higher level of physical activity, and individuals with a greater number of present diseases achieve a significantly lower level of physical activity, the existence of limitations and comorbidities were not recognized as significant predictors of physical activity (P < 0.05). Major depressive disorder, osteoporosis, recovered heart attack and cerebrovascular insult, sideropenic anaemia along with BMI and age are significant predictors of physical activity, showing a downward effect in the range from 0.68 to 0.98 (P < 0.05). Conclusion: The PA-DDF questionnaire is a reliable and valid instrument for assessing and monitoring changes in the physical activity of elderly in institutional care (nursing homes), thus ensuring its application in interdisciplinary gerontological care.