Background: The fulfilment of basic psychological needs (BPNs) is seen as an integral part of human self-determination, subjective wellbeing, and overall quality of life. However, the meaning of these psychological constructs for individuals with the most extensive support needs remains elusive. Methods: Primary relatives of nine people diagnosed with severe or profound intellectual and multiple disabilities were interviewed about their perceptions of autonomy, competence, and relatedness regarding their family member with complex care needs, and about the ways in which they tried to support their family member in fulfilling specific BPNs. The interview analysis followed a grounded theory with the sensitizing concepts approach. Results: The relatives assigned important meaning to the BPNs, providing insights into their subtle nature, their implicit drivers, and how they were experienced. The relatives also identified serious challenges in detecting, clarifying, and creating opportunities for BPNs. Conclusions: The themes in the relatives’ perspectives can be summarized into a conceptual framework that may contribute to better mutual understanding between people with complex care needs, their relatives, and healthcare providers.