We place tight constraints on the main cosmological parameters of spatially flat cosmological models by using the recent angular clustering results of XMM–Newton soft (0.5–2 keV) X-ray sources, which have a redshift distribution with a median of z ∼ 1. Performing a standard likelihood procedure, assuming a constant in comoving coordinates active galactic nuclei (AGN) clustering evolution, the AGN bias evolution model of Basilakos, Plionis & Ragone-Figueroa and the Wilkinson Microwave Anisotropy Probe5 value of σ₈, we find stringent simultaneous constraints in the (Ωₘ, w) plane, with Ωₘ= 0.26 ± 0.05, w =−0.93⁺⁰˙¹¹₋₀․₁₉.