Previously, a photoanode composed of nanostructured SnO₂ coated with the perylene diimide dye N,N′-bis(phosphonomethyl)-3,4,9,10-perylenediimide (PMPDI) plus photoelectrochemically deposited cobalt oxide (CoO_y) was shown to photoelectrochemically oxidize water at 31 ± 7% faradaic efficiency. A non-ideal part of that prior system is that the addition of the known CoO_y water oxidation catalyst (WOC) resulted in a reduction of the total photocurrent rather than the anticipated increase, due to an increase in charge-carrier recombination. Herein, we show deposition of an ultrathin alumina overlayer applied by atomic layer deposition (ALD) on the SnO₂/PMPDI photoanode can improve the photoactivity and catalytic activity of the system; the addition a ca. 1 nm-thick AlO_x layer deposited on a 4000 nm (i.e., 4 micron) thick mesoporous anode system can and does have a positive, 2.5-fold improvement in the steady-state photocurrent with 29 ± 9% faradaic efficiency vs. the control anode without alumina passivation by reducing charge-carrier recombination. Moreover, ALD-deposited AlO_x layer does help support the understanding of the “anti-catalysis” of co-depositing a CoO_y WOC on the SnO₂/PMPDI DS-PECs—specifically the picture of direct CoO_y–SnO₂ contact-mediated recombination—but that AlO_x layer was unable to improve the photocurrent in a net SnO₂/PMPDI/AlO_x(/CoO_y) system. We attribute the lack of a performance enhancement by CoO_y WOC to incomplete coverage of each SnO₂ nanoparticle by the AlO_x. Overall, we find the addition of an optimized ultrathin AlO_x layer (0.6 nm thick; deposited at 85 °C) improves the SnO₂/PMPDI/AlO_x system's photoactivity by a factor of up to ca. 3-fold with reduced recombination. These results document that metal-oxide passivation by low-temperature ALD can be an effective strategy for improving the water oxidation performance of even nanostructured dye sensitized-photoelectrochemical cell.