Photoelectrocatalysis offers a powerful method for driving organic transformations by combining light and electrical energy to access more extreme potentials to generate reactive intermediates under milder conditions. Herein, we report the first application of a metal halide perovskite photoelectrocatalytic system for C-(sp)-H activation, leading to C-C and C-N bond formation via alkylation. A lead-free, alloyed double perovskite, CsAgNaBiBr, was engineered as a relatively stable photoanode and used catalytically (0.5-1 mol %) under visible-light irradiation and ambient conditions with 9,10-diphenylanthracene as a cocatalyst to facilitate cross-coupling organic syntheses. Extensive characterization of the photoanode by electrochemical analyses, powder X-ray diffraction, ultraviolet photoelectron spectroscopy, and scanning electron microscopy confirmed the structural integrity and photoelectrocatalytic activity of the CsAgNaBiBr anode. Under optimal conditions, aliphatic substrates underwent C-H activation and coupling with electron-deficient SOMOphiles to afford C-C coupled products in up to 94% yield and C-N coupled products in up to 99% yield. Mechanistic studies, including isotope labeling, kinetic isotope effect competition experiments, and density functional theory calculations, reveal that the reaction proceeds via photogenerated •Br radicals that induce hydrogen atom transfer from the solvent molecules to generate nucleophilic alkyl radicals. These nucleophilic alkyl radicals then underwent polarity-matched addition to electron-deficient alkenes or azo compounds to form the desired products, with C-H activation of the substrate identified as the rate-determining step. This work demonstrates C-H functionalization and cross-coupling reactions under remarkably mild ambient conditions and establishes a new frontier for the application of metal halide perovskites in the photoelectrocatalytic activation and tandem transformation of small organic molecules.