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Whispering gallery mode (WGM) resonators are compelling optical devices; however, they are nearly unexplored in the terahertz (THz) domain. In this Letter, we report on THz WGMs in quartz glass bubble resonators with a subwavelength wall thickness. An unprecedented study of both the amplitude and phase of THz WGMs is presented. The coherent THz frequency domain measurements are in excellent agreement with a simple analytical model and results from numerical simulations. A high finesse of 9 and a quality (𝑄) factor exceeding 440 at 0.47 THz are observed. Due to the large evanescent field, the high 𝑄-factor THz WGM bubble resonators can be used as a compact, highly sensitive sensor in the intriguing THz frequency range.