IntroductionMandailing Coffee is internationally recognized, yet the sustainability of its local micro, small, and medium enterprise ecosystem remains constrained by limited access to finance, weak managerial and technological capabilities, fragmented market relationships, and uneven institutional support. This study examines lopo, a traditional Mandailing communal institution, as a culturally embedded source of social capital within this business ecosystem.ObjectivesThis study analyzes how lopo culture generates social capital, how Islamic economic values reinforce that social capital, and how their interaction supports collaboration, resource mobilization, and the resilience of the Mandailing Coffee micro, small, and medium enterprise ecosystem.MethodThe study employed a qualitative case study design in Panyabungan District, Mandailing Natal Regency, North Sumatra, Indonesia. Data were collected from 25 participants, including coffee farmers, business owners, traditional and religious leaders, Islamic microfinance representatives, local government officials, and academics or business facilitators. Data collection used semi-structured interviews, participant observation, and document analysis. The data were analyzed through data condensation, thematic organization, and conclusion drawing and verification.ResultsThe findings show that lopo functions as an informal socio-economic institution for market information exchange, deliberation, trust formation, reciprocal assistance, and business networking. Repeated interactions support verbal agreements, labor and equipment sharing, informal financing, collective marketing, and access to institutional support. Islamic economic values—amanah, ta'awun, ukhuwah, shura, and al-'adl—reinforce these relationships by promoting trustworthiness, mutual assistance, solidarity, participatory decision-making, and fairness. Together, these mechanisms strengthen stakeholder connectivity, collective problem-solving, and adaptive capacity within the Mandailing Coffee business ecosystem.ImplicationsThe findings suggest that micro, small, and medium enterprise development should strengthen locally legitimate institutions and connect them with financial institutions, government programs, markets, and knowledge networks without undermining their informal trust-based character.Originality/NoveltyThis study develops an integrated explanation linking lopo culture, social capital, Islamic economic values, and business ecosystem strengthening. It extends conventional social capital perspectives by showing how culturally embedded institutions and Islamic ethical values jointly shape collaborative economic practices in a rural Muslim coffee economy.