The paper presents a detailed assessment of the influence of supersaturation temperature on the development of the microstructure and mechanical properties of high-manganese austenitic TWIP (Twinning-Induced Plasticity) steel. Particular attention was paid to the analysis of changes occurring in the material's structure and their direct impact on strength and plasticity parameters depending on the applied treatment temperature. The average austenite grain diameter after supersaturation in water from 900°C to 1100°C changes from about 15 μm to about 132 μm. The static tensile test showed that the mechanical properties of the tested TWIP steel after supersaturation within a given temperature range changed as follows: yield strength (YS0.2) from 485 MPa to 345 MPa, ultimate tensile strength (UTS) from 797 MPa to 656 MPa, elongation (UEl) from 39% to 49%, and reduction in area (RA) from 43% to 51%. Transmission electron microscopy studies allowed to identify (Ti,Nb)C and (Ti,Nb)(C,N) type precipitations, mainly located at the austenite grain boundaries, ensures fine-grained microstructure. Microstructure observations with the use of X-ray and EBSD techniques, after supersaturation of the samples and their subsequent tensile testing, also showed the presence of  martensite, the average content of which – regardless of the supersaturation temperature – was approximately 8%. In the investigated steel, the presence of non-metallic inclusions was also revealed, mainly of the oxide–sulfide type, with an average diameter ranging from 1 μm to 3 μm. These inclusions were effectively modified by rare earth elements (La, Ce, Nd).