In an exclusive measurement of the reaction γd→K+K−pn, a narrow peak that can be attributed to an exotic baryon with strangeness S=+1 is seen in the K+n invariant mass spectrum. The peak is at 1.542±0.005  GeV/c2 with a measured width of 0.021  GeV/c2 FWHM, which is largely determined by experimental mass resolution. The statistical significance of the peak is (5.2±0.6)σ. The mass and width of the observed peak are consistent with recent reports of a narrow S=+1 baryon by other experimental groups.