HHP1 (heptahelical protein 1), a protein with a predicted seven transmembrane domain structure homologous to adiponectin receptors (AdipoRs) and membrane progestin receptors (mPRs), has been characterized. Expression of HHP1 was increased in response to abscisic acid (ABA) and salt/osmotic stress as shown by quantitative real-time PCR and HHP1 promoter-controlled GUS activity. The HHP1 T-DNA insertion mutant (hhp1-1) showed a higher sensitivity to ABA and osmotic stress than the wild-type (WT), as revealed by the germination rate and post-germination growth rate. The induced expression of stress-responsive genes (RD29A, RD29B, ADH1, KIN1, COR15A, and COR47) was more sensitive to exogenous ABA and osmotic stress in hhp1-1 than in the WT. The hypersensitivity in the hhp1-1 mutant was reversed in the complementation mutant of HHP1 expressing the HHP1 gene. The data suggest that the mutation of HHP1 renders plants hypersensitive to ABA and osmotic stress and HHP1 might be a negative regulator in ABA and osmotic signalling.