The blazar S4 0954+65 (at a disputed redshift of z=0.368 or z≥0.45) underwent an exceptionally high state in the optical band during January and February 2015, as revealed by the Tuorla and St. Petersburg University blazar monitoring programs: a brightening of more than 3 magnitudes in the R-band from the average monitored states. Simultaneous data from the Fermi satellite at high energy gamma rays (HE, 100 MeV < E < 100 GeV) also show a period of increased activity. MAGIC observations, triggered by the enhanced emission state at lower energies, led to the discovery of very high energy (VHE, E > 100 GeV) emission from S4 0954+65 (ATel #7080). The VHE flux above 150 GeV is estimated to be about 6% of the Crab nebula flux above the same threshold. In this contribution we present a comprehensive multiwavelength picture of this object, including data from mm/optical/X-ray/HE and VHE gamma-ray bands. The high emission state during the flare allows us to compile the simultaneous broadband spectral energy distribution and to characterize it in the scope of blazar jet emission models. The agreement of the broadband spectrum with an emission mechanism commonly invoked for flat spectrum radio quasars (i.e. inverse Compton scattering on an external soft photon field from the dust torus) is also discussed.