Leaf senescence, the final stage of leaf development, is regulated tightly by endogenous and environmental signals. MYBS3, a MYB transcription factor with a single DNA-binding domain, mediates sugar signaling in rice. Here we report that an Arabidopsis MYBS3 homolog, MYBH, plays a critical role in developmentally regulated and dark-induced leaf senescence by repressing transcription. Expression of MYBH was enhanced in older and dark-treated leaves. Gain- and loss-of-function analysis indicated that MYBH was involved in the onset of leaf senescence. Plants constitutively overexpressing MYBH underwent premature leaf senescence and showed enhanced expression of leaf senescence marker genes. In contrast, the MYBH mutant line, mybh-1, exhibited a delayed-senescence phenotype. The EAR repression domain was required for MYBH-regulated leaf senescence. Overexpression and knockout of MYBH repressed and enhanced auxin-responsive gene expression, respectively. MYBH repressed the auxin-amido synthase genes DFL1/GH3.6 and DFL2/GH3.10, which regulate auxin homoeostasis, by binding directly to the TA box in each of their regulatory regions. An auxin-responsive phenotype was enhanced in MYBH overexpression lines and reduced in mybh knockout lines. Overexpression of MYBH enhanced gene expression of SAUR36, an auxin-promoted leaf senescence key regulator, and accelerated ABA- and ethylene-induced leaf senescence in transgenic Arabidopsis plants. Our results suggest that the role of MYBH in controlling auxin homeostasis accounts for its capacity to participate in regulation of age- and darkness-induced leaf senescence in Arabidopsis.