  We present the first observation of a solar prominence at $84-116$ GHz using
the high resolution interferometric imaging of ALMA. Simultaneous observations
in H$\alpha$ from Bia{\l}kaw Observatory and with SDO/AIA reveal similar
prominence morphology to the ALMA observation. The contribution functions of 3
mm and H$\alpha$ emission are shown to have significant overlap across a range
of gas pressures. We estimate the maximum millimetre-continuum optical
thickness to be $\tau_\mathrm{3mm}\approx 2$, and the brightness temperature
from the observed H$\alpha$ intensity. The brightness temperature measured by
ALMA is $\sim 6000-7000$ K in the prominence spine, which correlates well with
the estimated brightness temperature for a gas temperature of 8000 K.
