Galaxy clusters provide a unique probe of the late-time cosmic structure and serve as a powerful independent test of the ΛCDM model. This work presents the first set of cosmological constraints derived with ∼16; 000 optically selected redMaPPer clusters across nearly 5000 deg² using DES year 3 datasets. Our analysis leverages a consistent modeling framework for galaxy cluster cosmology and DES-Y3 joint analyses of galaxy clustering and weak lensing (3 × 2pt), ensuring direct comparability with the DES-Y3 3 × 2pt analysis.We obtain constraints of $$ and $$ from the cluster-based data vector. We find that cluster constraints and 3 × 2pt constraints are consistent under the ΛCDM model with a posterior predictive distribution (PPD) value of 0.53. The consistency between clusters and 3 × 2pt provides a stringent test of ΛCDM across different mass and spatial scales. Jointly analyzing clusters with 3 × 2pt further improves cosmological constraints, yielding $$ and $$, a 24% improvement in the Ω<inf>m</inf> − S<inf>8</inf> figure of merit over 3 × 2pt alone. Moreover, we find no significant deviation from the Planck CMB constraints with a probability to exceed (PTE) value of 0.6, significantly reducing previous S<inf>8</inf> tension claims. Finally, combining DES 3 × 2pt, DES clusters, and PlanckCMBplaces an upper limit on the sum of neutrino masses of Pm<inf>ν</inf> < 0.26 eV at 95% confidence under the ΛCDM model. These results establish optically selected clusters as a key cosmological probe and pave the way for cluster-based analyses in upcoming stage-IV surveys such as LSST, Euclid, and Roman.