• Pseudomonas syringae type III effectors are known to suppress plant immunity to promote bacterial virulence. However, the activities and targets of these effectors are not well understood. • We used genetic, molecular, and cell biology methods to characterize the activities, localization, and target of the HopD1 type III effector in Arabidopsis. • HopD1 contributes to P. syringae virulence in Arabidopsis and reduces effector-triggered immunity (ETI) responses but not pathogen-associated molecular pattern-triggered immunity (PTI) responses. Plants expressing HopD1 supported increased growth of ETI-inducing P. syringae strains compared with wild-type Arabidopsis. We show that HopD1 interacts with the membrane-tethered Arabidopsis transcription factor NTL9 and demonstrate that this interaction occurs at the endoplasmic reticulum (ER). A P. syringae hopD1 mutant and ETI-inducing P. syringae strains exhibited enhanced growth on Arabidopsis ntl9 mutant plants. Conversely, growth of P. syringae strains was reduced in plants expressing a constitutively active NTL9 derivative, indicating that NTL9 is a positive regulator of plant immunity. Furthermore, HopD1 inhibited the induction of NTL9-regulated genes during ETI but not PTI. • HopD1 contributes to P. syringae virulence in part by targeting NTL9, resulting in the suppression of ETI responses but not PTI responses and the promotion of plant pathogenicity.