The cross-species spillover of coronaviruses is considered a serious public health risk. Feline coronavirus (FCoV), canine coronavirus (CCoV), and transmissible gastroenteritis virus (TGEV) are all classified under Alphacoronavirus suis and infect companion animals and livestock. Due to their frequent contact with humans, these viruses pose a potential risk of future cross-species transmission. Molnupiravir, a prodrug of N4-hydroxycytidine, exhibits potent antiviral activity against SARS-CoV-2, a member of the Betacoronavirus genus, and has been approved for the treatment of COVID-19. Molnupiravir was recently shown to be effective against FCoV, suggesting broad-spectrum antiviral activity across coronavirus lineages. Based on these findings, the present study investigated whether molnupiravir is also effective against CCoV and TGEV, which belong to the same Alphacoronavirus suis species as FCoV. We examined the in vitro antiviral effects of molnupiravir using four viral strains: FCoV-1 and -2, CCoV-2, and TGEV. Molnupiravir inhibited plaque formation, viral antigen expression, the production of infectious viral particles, and viral RNA replication in a dose-dependent manner in all strains. IC50 values for CCoV-2 and TGEV, calculated using a feline-derived cell line (fcwf-4), were significantly lower than those for FCoV, suggesting higher sensitivity to molnupiravir. These results demonstrate that molnupiravir exhibited broad antiviral activity against animal coronaviruses classified under Alphacoronavirus suis, providing a foundation for antiviral strategies to mitigate the future risk of cross-species transmission.