We report here the identification and characterization of VIGG, a novel virus-induced grapevine protein. Analysis of VIGG expression in grapevine demonstrated that VIGG was constitutively expressed in leaves and stems in virus-infected grapevine, and that VIGG expression was induced by grapevine virus A (GVA) infection, but not by infection with other viruses. The virus-induced expression profile of VIGG was supported by the finding that virus-free meristem cultures prepared from virus-infected grapevines did not express VIGG. An experiment using GFP-VIGG fusion protein demonstrated that VIGG might be localized in or around the endoplasmic reticulum (ER). Treatment of grapevine cells with ER stress inducers resulted in the induction of VIGG expression. Berries from VIGG-expressing grapevines had higher organic acid and phenolic contents than those from control grapevines that did not express VIGG. Interestingly, fruit composition of a grapevine that was simultaneously infected by GVA and grapevine virus B (GVB), which did not express VIGG, was significantly different from that of GVA-infected grapevines expressing VIGG, suggesting that the effector of fruit composition alteration might be VIGG expression, but not GVA infection. Taken together, VIGG expression might suppress the decrease in organic acid content and increase phenol content in berries. Further investigation of the biological function of VIGG is expected to provide new information on the fruit quality of grapevines.