Direct wavelength measurement of the 4p2 3P1−3P0 highly charged ion clock transition in Rh13+

•The 4p2 3P1−3P0 magnetic dipole clock transition in highly charged Rh13+ ions was firstly directly observed in an electron beam ion trap.•Using a dedicated internal calibration method, the line wavelength was determined at a few ppm precision level. The uncertainty was significantly improved by more than 350 time compared with the previous most precise indirect measurement.