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Herpesvirales are an ancient viral order that causes lifelong infections in species from mollusks to humans. They export their capsids from the nucleus to the cytoplasm by a noncanonical nuclear egress route that involves capsid budding at the inner nuclear membrane followed by fusion of this temporary envelope with the outer nuclear membrane. Here, using a whole-genome CRISPR screen, we identify ER protein CLCC1 as important for the fusion stage of nuclear egress in herpes simplex virus 1. We also find that the genomes of Herpesvirales that infect mollusks and fish encode CLCC1 genes acquired from host genomes by horizontal gene transfer. In uninfected cells, loss of CLCC1 causes a nuclear blebbing defect, suggesting a role in host nuclear export. We hypothesize that CLCC1 facilitates an ancient cellular membrane fusion mechanism that Herpesvirales have hijacked or co-opted for capsid export and propose a mechanistic model.