  New CCD photometric observations of UZ Leo were obtained between February
2012 and April 2013, and on February 2017. Its physical properties were derived
from detailed analyses of our light curves and existing radial velocities. The
results indicate that this system is a totally-eclipsing A-subtype overcontact
binary with both a high fill-out factor of 76 \% and a third light source
contributing 12 \% light in the $B$ bandpass, 10 \% in $V$, and 7 \% in $R$.
The light residuals between observations and theoretical models are
satisfactorily fitted by adopting a magnetic cool spot on the more massive
primary star. Including our 12 measurements, a total of 172 eclipse times were
used for ephemeris computations. We found that the orbital period of UZ Leo has
varied due to a periodic oscillation superposed on an upward parabolic
variation. The observed period increase at a rate of $+3.49 \times 10^{-7}$ d
yr$^{-1}$ can be plausibly explained by some combination of non-conservative
mass transfer from the secondary to the primary component and angular momentum
loss due to magnetic braking. The period and semi-amplitude of the oscillation
are about 139 yrs and 0.0225 d, respectively, which is interpreted as a
light-time effect due to a third component with a mass of $M_3 \sin i_3$ = 0.30
M$_\odot$. Because the third lights of 7$-$12 \% indicate that the circumbinary
object is very overluminous for its mass, it would possibly match a white
dwarf, rather than an M-type main sequence.
