Person-specific evidence was developed as a grounded theory by analyzing 20 selected case descriptions from interventions using the guided self-determination method with people with various long-term health conditions. It explains the mechanisms of mobilizing relational capacity by including person-specific evidence in shared decision-making. Person-specific self-insight was the first step, achieved as individuals completed reflection sheets enabling them to clarify their personal values and identify actions or omissions related to self-management challenges. This step paved the way for sharing these insights and challenges in a relationship with a supportive health professional, who could then rely on person-specific evidence instead of assumptions or a narrow disease perspective for shared decision-making. Trust in the evidence encouraged the supportive health professional to transfer it to the interdisciplinary team. Person-specific evidence then enhanced the ability of team members to apply general evidence in a meaningful way. The increased openness achieved by individuals through these steps enabled them to eventually share their new self-insights in daily life with other people, decreasing loneliness they experienced in self-management. Relational capacity, the core of the theory, is mobilized in both people with long-term health conditions and healthcare professionals. Further research on person-specific evidence and relational capacity in healthcare is recommended.