Calcium plays a pivotal role in plant responses to several stimuli, including pathogens, abiotic stresses, and hormones. However, the molecular mechanisms underlying calcium functions are poorly understood. It is hypothesized that calcium serves as second messenger and, in many cases, requires intracellular protein sensors to transduce the signal further downstream in the pathways. The calcineurin B-like proteins (CBLs) represent a unique family of calcium sensors in plant cells. Here, we report our analysis of the CBL9 member of this gene family. Expression of CBL9 was inducible by multiple stress signals and abscisic acid (ABA) in young seedlings. When CBL9 gene function was disrupted in Arabidopsis thaliana plants, the responses to ABA were drastically altered. The mutant plants became hypersensitive to ABA in the early developmental stages, including seed germination and post-germination seedling growth. In addition, seed germination in the mutant also showed increased sensitivity to inhibition by osmotic stress conditions produced by high concentrations of salt and mannitol. Further analyses indicated that increased stress sensitivity in the mutant may be a result of both ABA hypersensitivity and increased accumulation of ABA under the stress conditions. The cbl9 mutant plants showed enhanced expression of genes involved in ABA signaling, such as ABA-INSENSITIVE 4 and 5. This study has identified a calcium sensor as a common element in the ABA signaling and stress-induced ABA biosynthesis pathways.