Seed dormancy, a seed status that prohibits germination even in the presence of inductive germination signals, is a poorly understood process. To identify molecular components that regulate seed dormancy, we screened T-DNA insertion lines and identified a mutant designated honsu (hon). HON loss-of-function mutants display deep seed dormancy, whereas HON-overexpressing lines display shallow seed dormancy. HON encodes a seed-specific group A phosphatase 2C (PP2C) and is one of the major negative regulators of seed dormancy among group A PP2Cs. Like other PP2C family members, HON interacts with PYR1/RCAR11 in the presence of ABA. Our analysis indicates that HON inhibits ABA signaling and activates gibberellic acid signaling, and both of these conditions must be satisfied to promote the release of seed dormancy. However, HON mRNA levels are increased in mutants displaying deep seed dormancy or under conditions that deepen seed dormancy, and decreased in mutants displaying shallow seed dormancy or under conditions that promote the release of seed dormancy. Taken together, our results indicate that the expression of HON mRNA is homeostatically regulated by seed dormancy.