In plants, seasonal inputs such as photoperiod and temperature modulate the plant’s internal genetic program to regulate the timing of the developmental transition from vegetative to reproductive growth. This regulation of the floral transition involves chromatin remodeling, including covalent modification of histones. Here, we report that HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE 15 (HOS15), a WD40 repeat protein, associates with a histone deacetylase complex to repress transcription of the GIGANTEA (GI)-mediated photoperiodic flowering pathway in Arabidopsis (Arabidopsis thaliana). Loss of function of HOS15 confers early flowering under long-day conditions because elevated GI expression. LUX ARRHYTHMO (LUX), a DNA binding transcription factor and component of the Evening Complex (EC), is important for the binding of HOS15 to the GI promoter. In wild type, HOS15 associates with the EC components LUX, EARLY FLOWERING 3 (ELF3), and ELF4 and the histone deacetylase HDA9 at the GI promoter, resulting in histone deacetylation and reduced GI expression. In the hos15-2 mutant, the levels of histone acetylation are elevated at the GI promoter, resulting in increased GI expression. Our data suggest that the HOS15–EC–HDA9 histone-modifying complex regulates photoperiodic flowering via the transcriptional repression of GI.