Best-fit contours of w and Ω_m in the wCDM cosmological model from eROSITA experiment (orange), Planck CMB data (blue), SDSS BOSS DR16 BAO constraints (green), Pantheon SNe Ia constraints (brown), DES 3×2 constraints (grey) are shown. We also show the resulting combination of eROSITA and Pantheon SNe Ia (red), the combination of Planck CMB 2020 with Pantheon SNe Ia and BAO (purple), and finally, the combination of eROSITA with Planck CMB 2020, Pantheon SNe Ia and SDSS BOSS DR16 BAO (black). eRASS1 experiment can place tight constraints on the dark energy equation of state parameter without any need to combine with other cosmological experiments.