Knowledge sharing is crucial for the success of most R&D cooperations. This paper investigates the best conditions for fostering knowledge sharing in R&D cooperation and looks at how the establishment of Public-Private R&D Partnerships (PPP in R&D) could be a useful tool for this purpose. In this end, it proposes a theoretical model, related to the R&D cooperation literature, that takes into consideration the impacts of firms outside R&D cooperation and the presence of two kinds of spillover: a technology spillover and a product rivalry effect. The model shows that both spillovers can affect knowledge sharing negatively, and that PPP in R&D can be useful to promote knowledge sharing. First, public authorities can choose partners that will facilitate efficient knowledge sharing. Second, to avoid the negative impacts of spillovers on behavior in terms of knowledge sharing, public laboratories should be used as intermediaries for the prior and strategic knowledge of firms. Public labs can use the prior knowledge of firms to innovate, and then spread this innovation among the partners of the PPP, without spreading the prior knowledge of the firms.