  We report the observation of the doubly Cabibbo-suppressed decay $D^+\to K^+
\pi^- \pi^+$ in data from Fermilab charm hadroproduction experiment E791. With
a signal of 59 \pm 13 events we measured the ratio of the branching fraction
for this mode to that of the Cabibbo-favored decay $D^+\to K^- \pi^+ \pi^+$ to
be $B(D^+ \to K^+ \pi^- \pi^+) / B(D^+ \to K^- \pi^+ \pi^+) = (7.7 \pm 1.7 \pm
0.8) \times 10^{-3}$. A Dalitz plot analysis was performed to search for
resonant structures.
