  We present observations of Q1549-C25, an ~L* star-forming galaxy at z=3.15
for which Lyman-continuum (LyC) radiation is significantly detected in deep
Keck/LRIS spectroscopy. We find no evidence for contamination from a
lower-redshift interloper close to the line of sight in the high
signal-to-noise spectrum of Q1549-C25. Furthermore, the morphology of Q1549-C25
in V_606, J_125, and H_160 Hubble Space Telescope (HST) imaging reveals that
the object consists of a single, isolated component within 1". In combination,
these data indicate Q1549-C25 as a clean spectroscopic detection of LyC
radiation, only the second such object discovered to date at z~3. We model the
spectral energy distribution (SED) of Q1549-C25, finding evidence for
negligible dust extinction, an age (assuming continuous star formation) of ~1
Gyr, and a stellar mass of M_*=7.9x10^9 M_sun. Although it is not possible to
derive strong constraints on the absolute escape fraction of LyC emission,
f_esc(LyC), from a single object, we use simulations of intergalactic and
circumgalactic absorption to infer f_esc(LyC)>=0.51 at 95% confidence. The
combination of deep Keck/LRIS spectroscopy and HST imaging is required to
assemble a larger sample of objects like Q1549-C25, and obtain robust
constraints on the average f_esc(LyC) at z~3 and beyond.
