Global campaigns to control HIV, tuberculosis, malaria, and vaccine-preventable illnesses showed that large-scale impact can be achieved by using additional international financing to support selected, evidence-based, high-impact investment areas and to catalyse domestic resource mobilisation. Building on this paradigm, we make the case for targeting additional international funding for selected high-impact investments in primary health care. We have identified and costed a set of concrete, evidence-based investments that donors could support, which would be expected to have major impacts at an affordable cost. These investments are in: (1) individuals and communities empowered to engage in health decision making, (2) a new model of people-centred primary care, and (3) next generation community health workers. These three areas would be supported by strengthening two cross-cutting elements of national systems. The first is the digital tools and data that support facility, district, and national managers to improve processes, quality of care, and accountability across primary health care. The second is the educational, training, and supervisory systems needed to improve the quality of care. We estimate that with an additional international investment of between US$1·87 billion in a low-investment scenario and $3·85 billion in a high-investment scenario annually over the next 3 years, the international community could support the scale-up of this evidence-based package of investments in the 59 low-income and middle-income countries that are eligible for external financing from the World Bank Group's International Development Association.