As the dimensions of the semiconducting channels in field-effect transistors decrease, the contact resistance of the metal–semiconductor interface at the source and drain electrodes increases, dominating the performance of devices1,2,3. Two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS 2 ) have been demonstrated to be excellent semiconductors for ultrathin field-effect transistors4,5. However, unusually high contact resistance has been observed across the interface between the metal and the 2D transition-metal dichalcogenide3,5,6,7,8,9. Recent studies have shown that van der Waals contacts formed by transferred graphene10,11 and metals12 on few-layered transition-metal dichalcogenides produce good contact properties. However, van der Waals contacts between a three-dimensional metal and a monolayer 2D transition-metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10-nanometre-thick indium metal capped with 100-nanometre-thick gold electrodes and monolayer MoS 2 . Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid solution after annealing at 200 degrees Celsius and that the interface between the gold-capped indium and the MoS 2 is atomically sharp with no detectable chemical interaction between the metal and the 2D transition-metal dichalcogenide, suggesting van-der-Waals-type bonding between the gold-capped indium and monolayer MoS 2 . The contact resistance of the indium/gold electrodes is 3,000 ± 300 ohm micrometres for monolayer MoS 2 and 800 ± 200 ohm micrometres for few-layered MoS 2 . These values are among the lowest observed for three-dimensional metal electrodes evaporated onto MoS 2 , enabling high-performance field-effect transistors with a mobility of 167 ± 20 square centimetres per volt per second. We also demonstrate a low contact resistance of 220 ± 50 ohm micrometres on ultrathin niobium disulfide (NbS 2 ) and near-ideal band offsets, indicative of defect-free interfaces, in tungsten disulfide (WS 2 ) and tungsten diselenide (WSe 2 ) contacted with indium alloy. Our work provides a simple method of making ultraclean van der Waals contacts using standard laboratory technology on monolayer 2D semiconductors.