ABSTRACT This paper examines the effect of nuclear power plant decommissioning on electricity generation and carbon emissions in the United States. Using data on nuclear reactor closures in the United States between 1993 and 2022 and data on state‐level carbon emissions and electricity generation from the Energy Information Administration (EIA), this paper adopts a difference‐in‐differences (DiD) approach to estimate the effect of nuclear decommissioning. Two‐way fixed‐effects (TWFE) DiD estimates suggest a nuclear plant closure increases annual state‐level per capita carbon emissions between 6% and 8%. The increase in state‐level carbon emissions is driven by a substitution toward fossil fuel electricity—particularly coal‐fired electricity—following a plant closure. To address concerns about bias in TWFE estimates stemming from heterogeneous treatment effects and/or variation in treatment timing, I implement two alternative estimators robust to treatment effect heterogeneity. Estimates from both the Callaway and Sant'Anna (2021) and Borusyak et al. (2024) estimators support the TWFE findings: nuclear power plant closures increase per capita carbon emissions at the state level.