Abstract Seed dormancy and germination are dynamically regulated by abscisic acid (ABA), with transcription factor ABA INSENSITIVE 5 (ABI5) serving as a key repressor of germination under stress. Here, we identify QUIRKY (QKY), a MULTIPLE C2 DOMAIN AND TRANSMEMBRANE REGION PROTEIN (MCTP) in Arabidopsis, as a critical regulator of ABA signaling through posttranslational control of ABI5.