  The first direct measurement of the absolute branching fraction of $\Sigma^+
\to \Lambda e^+ \nu_{e}$ is reported based on an $e^+e^-$ annihilation sample
of $(10087\pm44) \times 10^6$ $J/\psi$ events collected with the BESIII
detector at $\sqrt{s}=3.097$ GeV. The branching fraction is determined to be
${\mathcal B}(\Sigma^+ \to \Lambda e^+ \nu_{e}) = [2.93\pm0.74(\rm stat) \pm
0.13(\rm syst)]\times 10^{-5}$, which is the most precise measurement obtained
in a single experiment to date and also the first result obtained at a collider
experiment. Combining this result with the world average of ${\mathcal
B}(\Sigma^- \to \Lambda e^- \bar{\nu}_{e})$ and the lifetimes of
$\Sigma^{\pm}$, the ratio, $\frac{\Gamma(\Sigma^- \to \Lambda e^-
\bar{\nu}_{e})}{\Gamma(\Sigma^+ \to \Lambda e^+ \nu_{e})}$, is determined to be
$1.06 \pm 0.28$, which is within 1.8 standard deviations of the value expected
in the absence of second-class currents that are forbidden in the Standard
Model.
