Timber and agricultural production must both increase throughout this century to meet rising demand. Understanding how climate-induced shifts in agricultural suitability will trigger competition with timber for productive land is crucial. Here, we combine predictions of agricultural suitability under different climate change scenarios (representative concentration pathways RCP 2.6 and RCP 8.5) with timber-production maps to show that 240–320 Mha (20–26%) of current forestry land will become more suitable for agriculture by 2100. Forestry land contributes 21–27% of new agricultural productivity frontiers (67–105 Mha) despite only occupying 10% of the surface of the land. Agricultural frontiers in forestry land occur disproportionately in key timber-producing nations (Russia, the USA, Canada and China) and are closer to population centres and existing cropland than frontiers outside forestry land. To minimize crop expansion into forestry land and prevent shifting timber harvests into old-growth tropical and boreal forests to meet timber demand, emissions must be reduced, agricultural efficiency improved and sustainable intensification invested in.