Lithium, the lightest metal, has long been considered to have a 'simple' electronic structure that can be well explained within the nearly-free-electron model. But lithium does not stay 'simple' under compression: rather than becoming more free-electron-like as pressure is increased, first-principles calculations1, 2 suggest that it transforms into a semi-metal or semiconductor. Experimentally, it has been shown that dense lithium adopts low-symmetry structures3, 4; there is also evidence that its resistivity increases with pressure5, 6, 7, 8. However, the electronic transport properties of lithium have so far not been directly monitored as a function of increasing static pressure. Here we report electrical resistance measurements on lithium in a diamond anvil cell up to pressures of 105 GPa, which reveal a significant increase in electrical resistivity and a change in its temperature dependence near 80 GPa. Our data thus provide unambiguous experimental evidence for a pressure-induced metal-to-semiconductor transition in a 'simple' metallic element.