  We present the first dedicated gamma-ray analysis of Jupiter, using 12 years
of data from the Fermi Telescope. We find no robust evidence of gamma-ray
emission, and set upper limits of $\sim10^{-9}~$GeV cm$^{-2} $s$^{-1}$ on the
Jovian gamma-ray flux. We point out that Jupiter is an advantageous dark matter
(DM) target due to its large surface area (compared to other solar system
planets), and cool core temperature (compared to the Sun). These properties
allow Jupiter to both capture and retain lighter DM, providing a complementary
probe of sub-GeV DM. Our analysis focuses on the annihilation of sub-GeV DM to
long-lived particles, which can escape the Jovian surface and decay into gamma
rays. In this regime, we constrain DM-proton scattering cross-sections as low
as $10^{-41}~$cm$^2$, which is up to ten orders of magnitude more sensitive
than direct detection, and subject to fewer astrophysical uncertainties than
other limits in this parameter space. Our work motivates follow-up studies with
upcoming MeV telescopes such as AMEGO and e-ASTROGAM.
