  We report the first absolute measurement of the branching fraction of
$\Lambda^+_{c}\rightarrow \Lambda e^+\nu_e$. This measurement is based on 567
pb$^{-1}$ of $e^+e^-$ annihilation data produced at $\sqrt{s}=4.599$ GeV, which
is just above the $\Lambda^+_c\bar{\Lambda}^-_c$ threshold. The data were
collected with the BESIII detector at the BEPCII storage rings. The branching
fraction is determined to be $\mathcal B({\Lambda^+_c\rightarrow \Lambda
e^+\nu_e})=(3.63\pm0.38({\rm stat})\pm0.20({\rm syst}))\%$, representing a more
than twofold improvement in precision upon previously published results. As the
branching fraction for $\Lambda^+_{c}\rightarrow \Lambda e^+\nu_e$ is the
benchmark for those of other $\Lambda^+_c$ semileptonic channels, our result
provides a unique test of different theoretical models, which is the most
stringent to date.
