The unfolded-protein response (UPR), activated by sensor molecules PERK, ATF6, and IRE1 to resolve endoplasmic reticulum (ER) stress, has emerged as a key target for host cells and viruses to control the infection outcomes. The UPR regulates ER protein folding, controls cell fate upon ER stress, and plays an important role in innate immunity. We and others have shown that human cytomegalovirus (HCMV) modulates the UPR. We show here that murine CMV (MCMV), the widely used CMV model for small animal infection, regulated the UPR in a manner similar to that of HCMV. This modulatory ability was triggered by virion entry and enhanced by viral immediate-early and early gene expression. Thus, while vulnerable at early times, MCMV became resistant to exogenous ER stress at late times of infection. MCMV activated the PERK-ATF4 pathway but only induced a subset of representative ATF4 targets at levels somewhat lower than those by the ER stress inducer tunicamycin. Moreover, MCMV induced ER chaperone Bip but actively blocked IRE1-mediated Xbp1(s) protein accumulation. ATF4 depletion severely attenuated viral growth at a low multiplicity of infection by modestly reducing viral DNA synthesis and more pronouncedly inhibiting late gene transcription. Collectively, we show that the UPR is a conserved target of CMVs and identify ATF4, a key UPR component, as a factor critical for MCMV infection. This work sets the stage for using the MCMV model to explore the role of this stress response in CMV biology, particularly during infection of the host, which is difficult to study in HCMV.