The Dark Energy Survey Year 1 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing presents cosmological constraints from a combined analysis of galaxy clustering and weak gravitational lensing using 1321 square degrees of griz imaging data from the first year of the Dark Energy Survey (DES Y1). The study combines three two-point functions: the cosmic shear correlation function of 26 million source galaxies in four redshift bins, the galaxy angular autocorrelation function of 650,000 luminous red galaxies in five redshift bins, and the galaxy-shear cross-correlation of luminous red galaxy positions and source galaxy shears. The analysis uses flat ΛCDM and wCDM cosmologies, marginalizing over 20 nuisance parameters and varying 6 (for ΛCDM) or 7 (for wCDM) cosmological parameters. The results show consistent cosmological constraints from these three two-point functions, with S8 = 0.773 ± 0.020 and Ωm = 0.267 ± 0.017 for ΛCDM, and S8 = 0.782 ± 0.024, Ωm = 0.284 ± 0.030, and w = -0.82 ± 0.20 for wCDM at 68% CL. The precision of these DES Y1 constraints rivals that from the Planck cosmic microwave background measurements, allowing a comparison of structure in the very early and late Universe on equal terms. Combining DES Y1 with Planck, Baryonic Acoustic Oscillation measurements, and type Ia supernovae data, the study derives very tight constraints on cosmological parameters: S8 = 0.802 ± 0.012 and Ωm = 0.298 ± 0.007 in ΛCDM, and w = -1.00 ± 0.04 in wCDM. Upcoming DES analyses will provide more stringent tests of the ΛCDM model and extensions such as a time-varying equation of state of dark energy or modified gravity. The study uses the Dark Energy Camera to image the South Galactic Cap in the grizY filters and analyzes DECam images taken from August 31, 2013 to February 9, 2014. The data are processed through the DES Data Management system, which detrends and calibrates the raw DES images, combines individual exposures to create coadded images, and detects and catalogs astrophysical objects. The study uses redMaGiC galaxies for galaxy clustering measurements and as the lens population for the galaxy–galaxy lensing analysis. The results show that the DES Y1 constraints are consistent with Planck dataThe Dark Energy Survey Year 1 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing presents cosmological constraints from a combined analysis of galaxy clustering and weak gravitational lensing using 1321 square degrees of griz imaging data from the first year of the Dark Energy Survey (DES Y1). The study combines three two-point functions: the cosmic shear correlation function of 26 million source galaxies in four redshift bins, the galaxy angular autocorrelation function of 650,000 luminous red galaxies in five redshift bins, and the galaxy-shear cross-correlation of luminous red galaxy positions and source galaxy shears. The analysis uses flat ΛCDM and wCDM cosmologies, marginalizing over 20 nuisance parameters and varying 6 (for ΛCDM) or 7 (for wCDM) cosmological parameters. The results show consistent cosmological constraints from these three two-point functions, with S8 = 0.773 ± 0.020 and Ωm = 0.267 ± 0.017 for ΛCDM, and S8 = 0.782 ± 0.024, Ωm = 0.284 ± 0.030, and w = -0.82 ± 0.20 for wCDM at 68% CL. The precision of these DES Y1 constraints rivals that from the Planck cosmic microwave background measurements, allowing a comparison of structure in the very early and late Universe on equal terms. Combining DES Y1 with Planck, Baryonic Acoustic Oscillation measurements, and type Ia supernovae data, the study derives very tight constraints on cosmological parameters: S8 = 0.802 ± 0.012 and Ωm = 0.298 ± 0.007 in ΛCDM, and w = -1.00 ± 0.04 in wCDM. Upcoming DES analyses will provide more stringent tests of the ΛCDM model and extensions such as a time-varying equation of state of dark energy or modified gravity. The study uses the Dark Energy Camera to image the South Galactic Cap in the grizY filters and analyzes DECam images taken from August 31, 2013 to February 9, 2014. The data are processed through the DES Data Management system, which detrends and calibrates the raw DES images, combines individual exposures to create coadded images, and detects and catalogs astrophysical objects. The study uses redMaGiC galaxies for galaxy clustering measurements and as the lens population for the galaxy–galaxy lensing analysis. The results show that the DES Y1 constraints are consistent with Planck data