Despite the well-documented health, social and economic benefits of sports participation, adults' participation in organised sport declines as age increases. To date, no review has summarised the multi-level factors that influence adults' decisions to participate in sport. Therefore, this systematic literature review aimed to: 1) determine the facilitators, constraints and negotiated constraints to adults' (25-64 years) sport participation, and 2) summarise these factors according to the multiple levels of the social-ecological model. A total of 91 articles were identified following an extensive literature search conducted according to the PRISMA guidelines. Studies were published over four decades (1983-2023), predominantly located in North America (n = 45; 49.5%), ranged from 5 to 10,646 participants, examined mainly middle-aged adults (M = 35-44 years; n = 52; 48.6%), included more males (61.6%) than females (38.1%), and were conducted in primarily single sport contexts (n = 63; 69.2%). Overall, more unique facilitators (55 items) were identified than constraints (35 items) and negotiated constraints (13 items). The desire for improved health and enjoyment were the most frequently reported facilitators, and the main constraints were injury or illness and family commitments. Constraint negotiation strategies largely included individual (e.g., implementing financial strategies) and interpersonal factors (e.g., encouraging others to participate). This review highlighted the expansive multi-level factors that influence adults' sports participation, reiterated the complexity of developing appropriate sport offerings, and identified a lack of studies examining non-sport participants. Strategies to support adult sport participation should focus on enjoyment, consider co-designed sport modifications to alleviate perceived constraints and integrate behaviour change theory to foster positive sport participation habits.