Abstract We present constraints on cosmological parameters from the Pantheon+ analysis of 1701 light curves of 1550 distinct Type Ia supernovae (SNe Ia) ranging in redshift from z = 0.001 to 2.26. This work features an increased sample size from the addition of multiple cross-calibrated photometric systems of SNe covering an increased redshift span, and improved treatments of systematic uncertainties in comparison to the original Pantheon analysis, which together result in a factor of 2 improvement in cosmological constraining power. For a flat ΛCDM model, we find Ω _M = 0.334 ± 0.018 from SNe Ia alone. For a flat w ₀CDM model, we measure w ₀ = −0.90 ± 0.14 from SNe Ia alone, H ₀ = 73.5 ± 1.1 km s⁻¹ Mpc⁻¹ when including the Cepheid host distances and covariance (SH0ES), and w ₀ = − 0.978 − 0.031 + 0.024 when combining the SN likelihood with Planck constraints from the cosmic microwave background (CMB) and baryon acoustic oscillations (BAO); both w ₀ values are consistent with a cosmological constant. We also present the most precise measurements to date on the evolution of dark energy in a flat w ₀ w _a CDM universe, and measure w _a = − 0.1 − 2.0 + 0.9 from Pantheon+ SNe Ia alone, H ₀ = 73.3 ± 1.1 km s⁻¹ Mpc⁻¹ when including SH0ES Cepheid distances, and w _a = − 0.65 − 0.32 + 0.28 when combining Pantheon+ SNe Ia with CMB and BAO data. Finally, we find that systematic uncertainties in the use of SNe Ia along the distance ladder comprise less than one-third of the total uncertainty in the measurement of H ₀ and cannot explain the present “Hubble tension” between local measurements and early universe predictions from the cosmological model.