We have synthesized ruthenium(II)– and osmium(II)–polypyridyl complexes ([M(bpy)2L]2+, in which M=OsII or RuII, bpy=2,2′‐bipyridyl, and L=4‐(2,2′‐bipyridinyl‐4‐yl)benzene‐1,2‐diol) and studied the interfacial electron‐transfer process on a TiO2 nanoparticle surface using femtosecond transient‐absorption spectroscopy. Ruthenium(II)‐ and osmium(II)‐based dyes have a similar molecular structure; nevertheless, we have observed quite different interfacial electron‐transfer dynamics (both forward and backward). In the case of the RuII/TiO2 system, single‐exponential electron injection takes place from photoexcited nonthermalized metal‐to‐ligand charge transfer (MLCT) states. However, in the case of the OsII/TiO2 system, electron injection takes place biexponentially from both nonthermalized and thermalized MLCT states (mainly 3MLCT states). Larger spin–orbit coupling for the heavier transition‐metal osmium, relative to that of ruthenium, accounts for the more efficient population of the 3MLCT states in the OsII‐based dye during the electron‐injection process that yields biexponential dynamics. Our results tend to suggest that appropriately designed OsII–polypyridyl dye can be a better sensitizer molecule relative to its RuII analogue not only due to much broader absorption in the visible region of the solar‐emission spectrum, but also on account of slower charge recombination.