2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/203082Chern-Simons modified gravity comprises the Einstein-Hilbert action and a higher-derivative interaction containing the Chern-Pontryagin density. We derive the analog of the Gibbons-Hawking-York boundary term required to render the Dirichlet boundary value problem well-defined. It turns out to be a boundary Chern-Simons action for the extrinsic curvature. We address applications to black hole thermodynamics.4 pages, revtex4, v2: added Refs., made one statement stronger, added footnote and added paragraph on single field inflationGeneral Relativity and Quantum CosmologyAstrophysicsHigh Energy Physics - Theorytext