  The Cabibbo-allowed weak radiative decay $\Lambda_{c}^+\to \Sigma^+\gamma$
has been searched for in a sample of $\Lambda_{c}^+\bar{\Lambda}_{c}^-$ pairs
produced in $e^+e^-$ annihilations, corresponding to an integrated luminosity
of $4.5 \mathrm{fb}^{-1}$ collected with the BESIII detector at center-of-mass
energies between 4.60 and 4.70 GeV. No excess of signal above background is
observed, and we set an upper limit on the branching fraction of this decay to
be ${\mathcal B}(\Lambda_{c}^+\to \Sigma^+\gamma)<4.4\times10^{-4}$ at a
confidence level of 90\%, which is in agreement with Standard Model
expectations.
