The unfolded protein response (UPR) controls cellular homeostasis and virus infections. It is canonically initiated by dissociation of ER chaperone BiP/GRP78 from lumenal domains of transmembrane sensor proteins, including the central UPR sensor inositol-requiring enzyme 1 alpha (IRE1a). This results in IRE1a activation and splicing of X-box binding protein 1 (XBP1) mRNA, leading to transcriptional activation of XBP1 for cellular adaptation and sustained infection. Human adenovirus (AdV) preferentially activates the IRE1a-XBP1 pathway through the viral membrane protein E3-19K, yet the mechanisms underlying this selective activation remain unclear. Here, we show that AdV uses two distinct early proteins to dissociate BiP from IRE1a and activate IRE1a-mediated XBP1 splicing. The immediate early protein E1A forms a complex with the cytoplasmic domain of IRE1a, sufficient to dissociate BiP from IRE1a without inducing XBP1 splicing or UPR activation. In contrast, E3-19K is expressed downstream of E1A, associates with the lumenal domain of IRE1a, and induces XBP1 splicing without promoting BiP dissociation from IRE1a. Both proteins were detected in IRE1a-containing complexes, consistent with stepwise viral regulation. Together, our findings show that canonical IRE1a activation hallmarks can be mechanistically separated during viral infection, revealing distinct AdV-regulated IRE1a activation states with discrete upstream and downstream signaling outputs.