We present Neutron Star Interior Composition Explorer X-ray and Arcminute Microkelvin Imager Large Array radio observations of a rapid hard-to-soft state transition in the black hole X-ray transient MAXI J1820+070. During the transition from the hard state to the soft state a switch between two particular types of quasiperiodic oscillations (QPOs) was seen in the X-ray power density spectra, from type-C to type-B, along with a drop in the strength of the broadband X-ray variability and a brief flare in the 7-12 keV band. Soon after this switch (∼1.5-2.5 hr) a strong radio flare was observed that corresponded to the launch of superluminal ejecta. Although hints of a connection between QPO transitions and radio flares have been seen in other black hole X-ray transients, our observations constitute the strongest observational evidence to date for a link between the appearance of type-B QPOs and the launch of discrete jet ejections. are usually recognized: the hard state, which shows strong rapid (subsecond) X-ray variability and a spectrum dominated by Comptonized power-law emission, and the soft state, in which rapid X-ray variability is weak and where a thermal disk blackbody component dominates the spectrum. The transitions between these two spectral states, during which sources are often classified as being in an intermediate state, are accompanied by strong evolution of the X-ray variability properties. As a source starts the transition from the hard state to the soft state, strong (type-C) low-frequency quasi-periodic oscillations (QPOs) are usually present in the power density spectra (see Wijnands et al. 1999;Remillard et al. 2002;Casella et al. 2005, for the various low-frequency QPO types). Their frequencies increase from ∼0.01 Hz to ∼10 Hz as the spectrum softens. At some point during the transition the type-C QPOs and arXiv:2003.01012v1 [astro-ph.HE]