The B s 0  → J/ψϕϕ decay is observed in pp collision data corresponding to an integrated luminosity of 3 fb−1 recorded by the LHCb detector at centre-of-mass energies of 7 TeV and 8 TeV. This is the first observation of this decay channel, with a statistical significance of 15 standard deviations. The mass of the B s 0 meson is measured to be 5367.08 ± 0.38 ± 0.15 MeV/c2. The branching fraction ratio ℬ(B s 0  → J/ψϕϕ)/ℬ(B s 0  → J/ψϕ) is measured to be 0.0115 ± 0.0012 − 0.0009 + 0.0005 . In both cases, the first uncertainty is statistical and the second is systematic. No evidence for non-resonant B s 0  → J/ψϕK + K − or B s 0  → J/ψK + K − K + K − decays is found.