  We analyze the phase curve of the short-period transiting hot Jupiter system
WASP-19, which was observed by the Transiting Exoplanet Survey Satellite
($TESS$) in Sector 9. WASP-19 is one of only five transiting exoplanet systems
with full-orbit phase curve measurements at both optical and infrared
wavelengths. We measure a secondary eclipse depth of $470^{+130}_{-110}$ ppm
and detect a strong atmospheric brightness modulation signal with a
semi-amplitude of $319\pm51$ ppm. No significant offset is detected between the
substellar point and the region of maximum brightness on the dayside. There is
also no significant nightside flux detected, which is in agreement with the
nightside effective blackbody temperature of $1090^{+190}_{-250}$ derived from
the published $Spitzer$ phase curves for this planet. Placing the eclipse depth
measured in the $TESS$ bandpass alongside the large body of previous values
from the literature, we carry out the first atmospheric retrievals of
WASP-19b's secondary eclipse spectrum using the SCARLET code. The retrieval
analysis indicates that WASP-19b has a dayside atmosphere consistent with an
isotherm at $T=2240\pm40$ K and a visible geometric albedo of $0.16\pm0.04$,
indicating significant contribution from reflected starlight in the $TESS$
bandpass and moderately efficient day-night heat transport.
