We present optical light curves , redshifts , and classifications for 365 Â spectroscopically confirmed Type Ia supernovae ( SNe Ia ) discovered by the Pan-STARRS1 ( PS1 ) Medium Deep Survey . We detail improvements to the PS1 SN photometry , astrometry and calibration that reduce the systematic uncertainties in the PS1 SN Ia distances . We combine the subset of 279 Â PS1 SN Ia ( 0.03 < z < 0.68 ) with useful distance estimates of SN Ia from SDSS , SNLS , various low-z and HST samples to form the largest combined sample of SN Ia consisting of a total of 1048 Â SN Ia ranging from 0.01 < z < 2.3 , which we call the ‘ Pantheon Sample ’ . When combining Planck 2015 CMB measurements with the Pantheon SN sample , we find \Omega _ { m } = 0.307 \pm 0.012 and w = -1.026 \pm 0.041 for the w CDMÂ model . When the SN and CMB constraints are combined with constraints from BAO and local H _ { 0 } measurements , the analysis yields the most precise measurement of dark energy to date : w _ { 0 } = -1.007 \pm 0.089 and w _ { a } = -0.222 \pm 0.407 Â for the w _ { 0 } w _ { a } CDMÂ model . Tension with a cosmological constant previously seen in an analysis of PS1 and low-z SNe has diminished after an increase of 2 \times in the statistics of the PS1 sample , improved calibration and photometry , and stricter light-curve quality cuts . We find the systematic uncertainties in our measurements of dark energy are almost as large as the statistical uncertainties , primarily due to limitations of modeling the low-redshift sample . This must be addressed for future progress in using SN Ia to measure dark energy .