A new interstellar molecular ion, H2COH+ (protonated formaldehyde), has been detected toward Sgr B2, Orion KL, W51, and possibly in NGC 7538 and DR21(OH). Six transitions were detected in Sgr B2(M). The 11,0--10,1 transition was detected in all sources listed above. Searches were also made toward the cold, dark clouds TMC-1 and L134N, Orion (3N, 1E), and a red giant, IRC +10216, without success. The excitation temperatures of H2COH+ are calculated to be 60--110 K, and the column densities are on the order of 1012--1014 cm-2 in Sgr B2, Orion KL, and W51. The fractional abundance of H2COH+ is on the order of 10-11 to 10-9, and the ratio of H2COH+ to H2CO is in the range 0.001--0.5 in these objects. The values in Orion KL seem to be consistent with the "early time" values of recent model calculations by Lee, Bettens, & Herbst, but they appear to be higher than the model values in Sgr B2 and W51 even if we take the large uncertainties of column densities of H2CO into account. We suggest production routes starting from CH3OH may play an important role in the formation of H2COH+.