We report the first Terrestrial Electron Beam (TEB) detected by the Atmosphere-Space Interactions Monitor (ASIM). It happened on September 16th, 2018. The ASIM Modular X and Gamma-ray Sensor (MXGS) recorded a two millisecond long event, with a softer spectrum than typically recorded for Terrestrial Gamma-ray Flashes (TGF). The lightning discharge associated to this event was found in the World-Wide Lightning Location Network (WWLLN) data, close to the northern footpoint of the magnetic field line passing by the International Space Station (ISS). Imaging from a GOES-R geostationary satellite shows that the source TGF was produced close to an overshooting top of a thunderstorm. Monte-Carlo simulations were performed to reproduce the observed lightcurve and energy spectrum. The event can be explained by the secondary electrons and positrons produced by the TGF (i.e. the TEB), even if about 5\% to 10\% of the detected counts may be due to direct TGF photons.
A source TGF with a Gaussian angular distribution with standard deviation between 21$^o$ and 29$^o$ was found to reproduce the measurement. Assuming an isotropic beaming within a cone, compatible half angles are between 29$^o$ and 43$^o$, in agreement with previous studies.
The number of required photons for the source TGF could be estimated for various assumption of the source (altitude of production, angular distribution), and is estimated between $10^{17}$ and $10^{18}$ photons, i.e. compatible with the current consensus.