Population growth and improving living standards are leading to a growing demand for beef. The understorey in the 19 million hectares of oil palm plantation worldwide represent a source of cattle feed, while also sparing additional land conversion for grazing. Research on cattle grazing under oil palm is novel and knowledge is lacking on long-term profitability and sustainability of the system, a major deterrent to cattle–oil palm farming system adoption. Here we review the current literature on cattle grazing in oil palm plantations, to highlight the knowns and unknowns of grazing impacts on the understorey, cattle and oil palm productivity, as well as economic implications. This review's major outcomes are the following: (i) although understorey utilization as feed is a major advantage of cattle–oil palm integrated farming systems, declining understorey productivity under ageing plantation indicates the need for alternative solutions for sustained cattle productivity over the plantation's lifecycle; (ii) oil palm yields were reported to increase under cattle integration but evidence was weak, both oil palm yields and the pathways through which oil palm yields are impacted by cattle grazing should be further explored; (iii) economic analyses showed cattle grazing under oil palm plantations was profitable for large-scale oil palm producers due to cattle sales, and returns were further enhanced by reductions in weeding and fertilizer costs; (iv) sustainable cattle grazing was found to be difficult in smallholder oil palm plantations due to land constraints and the current practice of free-grazing; options for improving grazing sustainability and profitability in these systems should be explored. This systematic review concluded that cattle integration in oil palm plantations has great potential but further research is required to develop locally specific productive and sustainable systems of grazed cattle integration with oil palm plantations.