Objectives: Undernutrition is a major health concern particularly in vulnerable older adults. The present study aimed to reveal the causes of undernutrition as reported by community-dwelling older adults. Design: Twenty-five semi-structured interviews and two focus group discussions were performed and analyzed. Setting: Community-dwelling. Participants: Older adults. Measurements: A questionnaire on demographics, Short Nutritional Assessment Questionnaire 65+ and interviews on the potential causes of undernutrition. Results: 33 older adults agreed to participate in the interviews and focus groups. Our findings indicate that a wide variety of causes of undernutrition, both modifiable and non-modifiable, were mentioned by the older adults. Many modifiable causes of undernutrition were reported in the mental, social or food & appetite theme, such as poor food quality provided by meal services, the inability to do groceries, loneliness and mourning. Non-modifiable causes included, forgetfulness, aging, surgery and hospitalization. Conclusions: This study provides guidance to better understand the underlying causes of undernutrition from an older adult’s perspective. The modifiable causes provide specific direction towards practical implications that might decrease or prevent undernutrition. Non-modifiable causes should raise awareness of an increased risk of undernutrition by health professionals in primary and secondary care, caregivers and family members.