  The radiative decay $\Lambda_{b}^{0} \to \Lambda \gamma$ is observed for the
first time using a data sample of proton-proton collisions corresponding to an
integrated luminosity of $1.7\,fb^{-1}$ collected by the LHCb experiment at a
center-of-mass energy of $13\,TeV$. Its branching fraction is measured
exploiting the $B^{0} \to K^{*0} \gamma$ decay as a normalization mode and is
found to be $\mathcal{B}(\Lambda_{b}^{0} \to \Lambda \gamma)= (7.1 \pm 1.5 \pm
0.6 \pm 0.7) \times 10^{-6}$, where the quoted uncertainties are statistical,
systematic and systematic from external inputs, respectively. This is the first
observation of a radiative decay of a beauty baryon.
