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Abstract High energy photons (λ < 400 nm) are frequently used to initiate free radical polymerizations to form polymer networks, but are only effective for transparent objects. This phenomenon poses a major challenge to additive manufacturing of particle‐reinforced composite networks since deep light penetration of short‐wavelength photons limits the homogeneous modification of physicochemical and mechanical properties. Herein, the unconventional, yet versatile, multiexciton process of triplet–triplet annihilation upconversion (TTA‐UC) is employed for curing opaque hydrogel composites created by direct‐ink‐write (DIW) 3D printing. TTA‐UC converts low energy red light (λ_max = 660 nm) for deep penetration into higher‐energy blue light to initiate free radical polymerizations within opaque objects. As proof‐of‐principle, hydrogels containing up to 15 wt.% TiO₂filler particles and doped with TTA‐UC chromophores are readily cured with red light, while composites without the chromophores and TiO₂loadings as little as 1–2 wt.% remain uncured. Importantly, this method has wide potential to modify the chemical and mechanical properties of complex DIW 3D‐printed composite polymer networks.