  We present a measurement of the $B$-mode polarization power spectrum of the
cosmic microwave background (CMB) using taken from July 2014 to December 2016
with the POLARBEAR experiment. The CMB power spectra are measured using
observations at 150 GHz with an instantaneous array sensitivity of
$\mathrm{NET}_\mathrm{array}=23\, \mu \mathrm{K} \sqrt{\mathrm{s}}$ on a 670
square degree patch of sky centered at (RA,
Dec)=($+0^\mathrm{h}12^\mathrm{m}0^\mathrm{s},-59^\circ18^\prime$). A
continuously rotating half-wave plate is used to modulate polarization and to
suppress low-frequency noise. We achieve $32\,\mu\mathrm{K}$-$\mathrm{arcmin}$
effective polarization map noise with a knee in sensitivity of $\ell = 90$,
where the inflationary gravitational wave signal is expected to peak. The
measured $B$-mode power spectrum is consistent with a $\Lambda$CDM lensing and
single dust component foreground model over a range of multipoles $50 \leq \ell
\leq 600$. The data disfavor zero $C_\ell^{BB}$ at $2.2\sigma$ using this
$\ell$ range of POLARBEAR data alone. We cross-correlate our data with Planck
high frequency maps and find the low-$\ell$ $B$-mode power in the combined
dataset to be consistent with thermal dust emission. We place an upper limit on
the tensor-to-scalar ratio $r < 0.90$ at 95% confidence level after
marginalizing over foregrounds.
