Recently, sustainable partnerships between professional sports clubs and companies are being explored, with keywords such as “sponsorship activation” and “ESG (Environmental, Social and Governance) investment” being used. This study aims to clarify the factors that influence the formation and enhancement of partnerships between professional football clubs in the J. League and their sponsors (official partners). We conducted a case study on the official partner of Mito Hollyhock. Several qualitative research methods, such as document collection, interviews, and participant observation, were employed to trace the process of the formation and strengthening of the partnership. The results highlight that the partnership was realized through two factors： creating legitimacy for resource mobilization by the official partner, and building empathy for the club. The coexistence of various reasons； the contribution of an opinion leader, and the organizational characteristics of the official partner (structure, culture, and size), were influential in the justification process of resource mobilization for the official partner. Furthermore, the establishment of the club’s brand promise, the implementation of management reforms, and the establishment of a brand image generated empathy from the official partner. This study suggests that partnerships between professional sports clubs and their sponsors are formed not only through the exchange of benefits, but also through co-creation.