We report an angle-resolved photoelectron spectroscopy study of the layered semiconductor ZrSe₂, and of changes in its electronic structure induced by in situ intercalation with Cs. The results show that the valence band structure of ZrSe₂ is initially of 3D character, but is transformed to become essentially 2D upon Cs intercalation. The observed changes are supported by self-consistent LAPW band calculations, and are not compatible with the rigid-band model. Changes in the Se 3d core level lineshape are attributed to an intercalation-induced increase in the carrier density in the lowest conduction band, which produces a different screening of the core hole.