The unfolded protein response (UPR) is a eukaryotic transcriptional regulatory network in response to accumulated malformed proteins in the endoplasmic reticulum (ER). In Arabidopsis thaliana, three bZIP transcription factors are dedicated to modulate the UPR: bZIP17, bZIP28, and bZIP60. Compared to bZIP28 and bZIP60 that have been relatively well studied, the physiological and transcriptional roles of bZIP17 remain largely unknown. In this report, we introduce a double knockout mutant of bZIP17 and bZIP28 to analyze the function of bZIP17. The mutant plant exhibited multiple developmental defects including scarce root elongation and constantly over-induced bZIP60 activity, indicating the essential roles of bZIP17 and bZIP28 in plant development and UPR modulation. Extended analysis on the transcriptome of the three double knockout mutants of bZIP17, bZIP28, and bZIP60 genes revealed that bZIP28 and bZIP60 majorly activated the canonical UPR in response to stimuli. In contrast, bZIP17 conferred the non-inducible expression of multiple genes involved in cell growth together with bZIP28, particularly to sustain their expression under stressed conditions. Our study revealed the pivotal roles of bZIP17 on plant UPR and vegetative development in functional redundancy with bZIP28.