The control of light–matter coupling at the single electron level is currently a subject of growing interest for the development of novel quantum devices and for studies and applications of quantum electrodynamics. In the terahertz (THz) spectral range, this raises the particular and difficult challenge of building electromagnetic resonators that can conciliate low mode volume and high quality factor. Here, we report on hybrid THz cavities based on ultrastrong coupling between a Tamm cavity and an LC circuit metamaterial and show that they can combine high quality factors of up to Q =37 with a deep-subwavelength mode volume of V =3.2×10 −4 λ 3 . Our theoretical and experimental analysis of the coupled mode properties reveals that, in general, the ultrastrong coupling between a metamaterial and a Fabry–Perot cavity is an effective tool to almost completely suppress radiative losses and, thus, ultimately limit the total losses to the losses in the metallic layer. These Tamm cavity-LC metamaterial coupled resonators open a route toward the development of single photon THz emitters and detectors and to the exploration of ultrastrong THz light–matter coupling with a high degree of coherence in the few to single electron limit.