  This paper reports the first results of a direct dark matter search with the
DEAP-3600 single-phase liquid argon (LAr) detector. The experiment was
performed 2 km underground at SNOLAB (Sudbury, Canada) utilizing a large target
mass, with the LAr target contained in a spherical acrylic vessel of 3600 kg
capacity. The LAr is viewed by an array of PMTs, which would register
scintillation light produced by rare nuclear recoil signals induced by dark
matter particle scattering. An analysis of 4.44 live days (fiducial exposure of
9.87 tonne-days) of data taken with the nearly full detector during the initial
filling phase demonstrates the detector performance and the best electronic
recoil rejection using pulse-shape discrimination in argon, with leakage
$<1.2\times 10^{-7}$ (90% C.L.) between 16 and 33 keV$_{ee}$. No candidate
signal events are observed, which results in the leading limit on WIMP-nucleon
spin-independent cross section on argon, $<1.2\times 10^{-44}$ cm$^2$ for a 100
GeV/c$^2$ WIMP mass (90% C.L.).
