BACKGROUND: Public health facilities often manage high volumes of HIV clients and face challenges with long wait times, poor client satisfaction, and low retention in care. The private sector is a major provider of health services in many countries but is a relatively untapped resource for reducing congestion in high-volume public HIV facilities and increasing access to HIV services, particularly for clients who already use the private sector as their first point of care. Private banks, corporations, and clients themselves can also be a resource for generating additional financing for HIV and reducing reliance on donor funding. Through the President's Emergency Plan for AIDS Relief's Sustainable Financing Initiative (SFI), USAID engaged the private sector to expand access to HIV care and mobilize private financing for HIV in low- and middle-income countries. METHODS: SFI worked in 11 countries and two regional programs on private sector activities, including technical assistance to financial institutions and private providers on lending to the health sector, facilitating the growth of social enterprise models, enabling private providers to offer additional HIV services, and generating and communicating evidence to host governments to strengthen the policy and regulatory environment for private HIV service delivery. SFI's impact was estimated through a financial return on investment. RESULTS: Over 48,000 clients accessed HIV services in the private sector across three countries, and $6.3 million was mobilized in private loans to the health sector in Tanzania. Additional successes in policy, evidence generation, innovative financing, and service delivery were demonstrated globally. LESSONS LEARNED AND RECOMMENDATIONS: SFI demonstrated the feasibility of delivering high-quality HIV services through the private sector and the high demand for those services; the need for evaluation techniques that incorporate patient experience in addition to program costs, benefits, and clinical outcomes; and the need for long-term strategic donor and government coordination around private sector engagement. CONCLUSIONS: Engaging the private sector in HIV can reduce the burden on public health facilities, provide greater opportunities for access to care, and increase domestic financing for HIV without putting clients into financial hardship.