Plant and animal pathogens inject type III effectors (T3Es) into host cells to suppress host immunity and promote successful infection. XopD, a T3E from Xanthomonas campestris pv vesicatoria, has been proposed to promote bacterial growth by targeting plant transcription factors and/or regulators. Here, we show that XopD from the B100 strain of X. campestris pv campestris is able to target MYB30, a transcription factor that positively regulates Arabidopsis thaliana defense and associated cell death responses to bacteria through transcriptional activation of genes related to very-long-chain fatty acid (VLCFA) metabolism. XopD specifically interacts with MYB30, resulting in inhibition of the transcriptional activation of MYB30 VLCFA-related target genes and suppression of Arabidopsis defense. The helix-loop-helix domain of XopD is necessary and sufficient to mediate these effects. These results illustrate an original strategy developed by Xanthomonas to subvert plant defense and promote development of disease.