BBX transcription factors are known to regulate the flowering time and the plant response to various abiotic stresses, but their functions in chrysanthemum have yet to be thoroughly explored. Here, a chrysanthemum homolog of the Arabidopsis thaliana gene AtBBX29 was isolated and characterized. The gene was transcribed in various plant organs but most strongly in the root and in the ligulate flowers. Its temporal pattern of transcription mirrored that of CmCO, the chrysanthemum homolog of the key flowering regulator CONSTANS (CO). Its constitutive expression in A. thaliana induced a delay to flowering, suppressing the transcription of FLOWERING LOCUS T (FT), SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), LEAFY (LFY), and APETALA 1 (AP1), while promoting that of FLOWERING LOCUS C (FLC). Our results indicate that CmBBX29 can regulate flowering time in A. thaliana by the integration of FLC and the photoperiod pathway.