  Dwarf spheroidal galaxies (dSphs) are promising targets for the indirect
detection of dark matter through gamma-ray emission due to their proximity,
lack of astrophysical backgrounds and high dark matter density. They are often
used to place restrictive bounds on the dark matter annihilation cross section.
In this paper, we analyze six years of {\it Fermi}-LAT gamma-ray data from 19
dSphs that are satellites of the Milky Way, and derive from a stacked analysis
of 15 dSphs, robust 95\% confidence level lower limits on the dark matter
lifetime for several decay channels and dark matter masses between $\sim 1$GeV
and $10$TeV. Our findings are based on a bin-by-bin maximum likelihood analysis
treating the J-factor as a nuisance parameter using PASS 8 event-class. Our
constraints from this ensemble are among the most stringent and solid in the
literature, and competitive with existing ones coming from the extragalactic
gamma-ray background, galaxy clusters, AMS-02 cosmic ray data, Super-K and
ICECUBE neutrino data, while rather insensitive to systematic uncertainties. In
particular, among gamma-ray searches, we improve existing limits for dark
matter decaying into $\bar{b}b$, ($\mu^+\mu^-$) for DM masses below $\sim 30\,
(200)$~GeV, demonstrating that dSphs are compelling targets for constraining
dark matter decay lifetimes.
