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Ionising radiation causes a wide variety of effects in biological targets. In experimental microdosimetry these effects are studied by investigating the statistical distribution of energy deposition events at the microscopic level employing a Tissue-Equivalent Proportional Counter (TEPC). A TEPC is usually designed with a cavity of a few centimetres in diameter containing a central anode wire and operated with a tissue-equivalent wall and counting gas at low pressure to simulate tissue volumes of micrometer dimensions. However, in experimental microdosimetry there are some conditions such as high flux therapy beams and energy deposition on nanometer level (nanodosimetry) in which TEPCs with very small cavities are needed. The construction of a central wire in a small cavity is extremely difficult and expensive. It becomes even more difficult when multi-element cavities are applied to increase the counter sensitivity. Therefore, a new type of TEPC was developed based on a Gas Electron Multiplier (GEM). The design and construction of a TEPC-GEM is described. The successful operation of this counter has been shown by the first microdosimetric measurements of the TEPC-GEM exposed to a 14 MeV monoenergetic neutron beam and a californium (252Cf) source for a cavity diameter of 1.8 mm, simulating a 1.0 μm tissue site size. The measured spectra show an excellent agreement with spectra from the literature. Furthermore, the first microdosimetric responses for low energy X-rays are presented for counter cavities of 1.8 mm, 1.0 mm and 0.5 mm diameter at various pressures, simulating tissue site sizes down to 140 nm. This new design not only simplifies the construction of mini multi-element counters but it also facilitates microdosimetry measurements (e.g. in nanodosimetry, personal dosimetry and space dosimetry) in a relatively easy manner. Because of the relatively simple design and construction, the TEPC-GEM is not only relatively cheap and easy to produce but also its components (sensitive volume, GEM and wall materials) can be modified and adapted to the needs. In general, it can be concluded that this small counter cavity opens new possibilities in application for high intensity radiation fields as well as in nanodosimetry. Furthermore, its robust and multi-element concept makes it suitable for applications in space and aviation.