We report the first observation of the doubly Cabibbo-suppressed decay Ds+→K+K+π- using 605fb-1 of data collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. The branching ratio with respect to its Cabibbo-favored counterpart B(Ds+→K+K+π-)/B(Ds+→K+K-π+) is (0.229±0.028±0.012)%, where the first uncertainty is statistical and the second is systematic. We also report a significantly improved measurement of the doubly Cabibbo-suppressed decay D+→K+π+π-, with a branching ratio B(D+→K+π+π-)/B(D+→K-π+π+)=(0.569±0. 018±0.014)%. © 2009 The American Physical Society.