In the last ten years, a whole set of new particles not fitting in the constituent quark model has populated the mass region above the open-charm threshold. The Beijing Spectrometer (BESIII), installed at the Beijing Electron Positron Collider (BEPCII), can access these states both in electron-positron annihilations and from the decays of charmonium(-like) states. In this proceeding, the discovery of the $Z_{cs}(3985)^-$ at BESIII with a mass of $3982.5^{+1.8}_{-2.6}\pm2.1$ MeV$/c^2$ will be addressed. The resonance was observed by analyzing data collected at five center-of-mass energy points in the range from 4.628 GeV to 4.698 GeV, with a total integrated luminosity of $3.7~\rm{fb}^{-1}$. Due to its properties, the $Z_{cs}(3985)^-$ is a strong candidate for the predicted open-strange charmonium-like tetraquark with the minimal structure $c\bar{c}s\bar{u}$.