Abstract We report that the RS CVn–type star GT Mus (HR 4492, HD 101379+HD 101380) was the most active star in the X-ray sky in the last decade in terms of the scale of recurrent energetic flares. We detected 11 flares from GT Mus in 8 yr of observations with the Monitor of All-sky X-ray Image (MAXI) from 2009 August to 2017 August. The detected flare peak luminosities were 1–4 × 10³³ erg s⁻¹ in the 2.0–20.0 keV band for its distance of 109.6 pc. Our timing analysis showed long durations (τ _r + τ _d) of 2–6 days with long decay times (τ _d) of 1–4 days. The released energies during the decay phases of the flares in the 0.1–100 keV band were in the range of 1–11 × 10³⁸ erg, which are at the upper end of the observed stellar flare. The released energies during the whole duration were in the range of 2–13 × 10³⁸ erg in the same band. We carried out X-ray follow-up observations for one of the 11 flares with the Neutron star Interior Composition Explorer (NICER) on 2017 July 18 and found that the flare cooled quasi-statically. On the basis of a quasi-static cooling model, the flare loop length is derived to be 4 × 10¹² cm (or 60 R _☉). The electron density is derived to be 1 × 10¹⁰ cm⁻³, which is consistent with the typical value of solar and stellar flares (10^10–13 cm⁻³). The ratio of the cooling timescales between radiative (τ _rad) and conductive (τ _cond) cooling is estimated to be τ _rad ∼ 0.1 τ _cond from the temperature; thus, radiative cooling was dominant in this flare.