This study demonstrates the first reported photocatalytic decomposition of an indoor air pollutant, acetaldehyde, using low-energy, sub-bandgap photons harnessed through sensitized triplet-triplet annihilation (TTA) upconversion (UC). To utilize low-intensity noncoherent indoor light and maximize photocatalytic activity, we designed a plasmon-enhanced sub-bandgap photocatalyst device consisting of two main components: (1) TTA-UC rubbery polymer films containing broad-band plasmonic particles (Ag-SiO2) to upconvert sub-bandgap photons, and (2) nanodiamond (ND)-loaded WO3 as a visible-light photocatalyst composite. Effective decomposition of acetaldehyde was achieved using ND/WO3 (Eg = 2.8 eV) coupled with TTA-UC polymer films that emit blue photons (λEm = 425 nm, 2.92 eV) upconverted from green photons (λEx = 532 nm, 2.33 eV), which are wasted in most environmental photocatalysis. The overall photocatalytic efficiency was amplified by the broad-band surface plasmon resonance of AgNP-SiO2 particles incorporated into the TTA-UC films.