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. (shrink)