  The specific activity of the beta decay of $^{39}$Ar in atmospheric argon is
measured using the DEAP-3600 detector. DEAP-3600, located 2 km underground at
SNOLAB, uses a total of (3269 $\pm$ 24) kg of liquid argon distilled from the
atmosphere to search for dark matter. This detector with very low background
uses pulseshape discrimination to differentiate between nuclear recoils and
electron recoils and is well-suited to measure the decay of $^{39}$Ar. With 167
live-days of data, the measured specific activity at the time of atmospheric
extraction is [0.964 $\pm$ 0.001 (stat) $\pm$ 0.024 (sys)] Bq/kg$_{\rm atmAr}$
which is consistent with results from other experiments. A cross-check analysis
using different event selection criteria provides a consistent result.
