We have previously shown that the Arabidopsis (Arabidopsis thaliana) RING-H2 E3 ligase RHA2a positively regulates ABA signaling during seed germination and postgerminative growth. Here we report that RHA2b, the closest homologue of RHA2a, is also an active E3 ligase and plays an important role in ABA signaling. We show that RHA2b expression is induced by ABA and overexpression of RHA2b leads to ABA-associated phenotypes such as ABA hypersensitivity in seed germination and seedling growth, enhanced stomatal closure, reduced water loss and, therefore, increased drought tolerance. On the contrary, the rha2b-1 mutant shows ABA-insensitive phenotypes and reduced drought tolerance. We provide evidence showing that a rha2a rha2b-1 double mutant generally enhances ABA insensitivity of rha2b-1 in seed germination, seedling growth and stomatal closure, suggesting that RHA2b and RHA2a act redundantly in regulating ABA responses. Genetic analyses support that, like RHA2a, the RHA2b action in ABA signaling is downstream of the PP2C phosphatase ABI2 and in parallel with that of the ABI transcription factors ABI3/4/5. We speculate that RHA2b and RHA2a may have redundant yet distinguishable functions in the regulation of ABA responses.