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Owing to their attractive optical and chiroptical properties, chiral metal halide perovskites have received increasing attention, with potential applications ranging from photonics and optoelectronics to spintronics. Metal halide perovskite nanocrystals with either intrinsic or extrinsic (e.g., chiral ligand‐induced) chirality have been reported recently, and the interplay between these two types of chirality has yet to be addressed. Herein, the inversion and tuning of excitonic optical activity is reported in intrinsically chiral perovskite nanoplatelets, originating from interactions between their structural chirality (due to the spontaneously formed screw dislocations in the crystalline lattice) and the surface enantiomeric (R/S) chiral ligands R/S‐phenylethylammonium bromide. Through post‐preparative exposure of the perovskite nanoplatelets to these R/S ligands of varied contents, either chiral ligand‐induced intrinsic chirality inversion or negative and positive Cotton effects induced by the ligands via electronic coupling between the ligand and the nanoplatelets are identified. These findings deepen understanding of the modulation of excitonic optical activity in chiral perovskites and can guide the rational design and synthesis of novel chiral materials. [ABSTRACT FROM AUTHOR]