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We report our identification of the optical afterglow and host galaxy of theshort-duration gamma-ray burst GRB 160821B. The spectroscopic redshift of thehost is $z=0.162$, making it one of the lowest redshift sGRBs identified bySwift. Our intensive follow-up campaign using a range of ground-basedfacilities as well as HST, XMM and Swift, shows evidence for a late-time excessof optical and near-infrared emission in addition to a complex afterglow. Theafterglow light-curve at X-ray frequencies reveals a narrow jet,$\theta_j\sim1.9^{+0.10}_{-0.03}$ deg, that is refreshed at $>1$ day post-burstby a slower outflow with significantly more energy than the initial outflowthat produced the main GRB. Observations of the 5 GHz radio afterglow shows areverse shock into a mildly magnetised shell. The optical and near-infraredexcess is fainter than AT2017gfo associated with GW170817, and is wellexplained by a kilonova with dynamic ejecta mass $M_{\rmdyn}=(1.0\pm0.6)\times10^{-3}$ M$_{\odot}$ and a secular (postmerger) ejectamass with $M_{\rm pm}=(1.0\pm0.6)\times10^{-2}$ M$_\odot$, consistent with abinary neutron star merger resulting in a short-lived massive neutron star.This optical and near-infrared dataset provides the best-sampled kilonovalight-curve without a gravitational wave trigger to date.

Short GRB 160821B: a reverse shock, a refreshed shock, and a well-sampled kilonova