  The singly Cabibbo-suppressed decay $\Lambda_{c}^{+} \to n\pi^{+}$ is
observed for the first time with a statistical significance of $7.3\sigma$ by
using 3.9 $\mathrm{fb}^{-1}$ of $e^{+}e^{-}$ collision data collected at
center-of-mass energies between 4.612 and 4.699 GeV with the BESIII detector at
BEPCII. The branching fraction of $\Lambda_{c}^{+} \to n\pi^{+}$ is measured to
be $(6.6\pm1.2_{\rm stat}\pm0.4_{\rm syst})\times 10^{-4}$. By taking the upper
limit of branching fractions of $\Lambda_{c}^{+} \to p\pi^0$ from the Belle
experiment, the ratio of branching fractions between $\Lambda_{c}^{+} \to
n\pi^{+}$ and $\Lambda_{c}^{+} \to p\pi^0$ is calculated to be larger than 7.2
at the 90% confidence level, which disagrees with the current predictions of
available phenomenological models. In addition, the branching fractions of the
Cabibbo-favored decays $\Lambda_{c}^{+} \to \Lambda\pi^{+}$ and
$\Lambda_{c}^{+} \to \Sigma^{0}\pi^{+}$ are measured to be $(1.31\pm0.08_{\rm
stat}\pm0.05_{\rm syst})\times 10^{-2}$ and $(1.22\pm0.08_{\rm
stat}\pm0.07_{\rm syst})\times 10^{-2}$, respectively, which are consistent
with previous results.
