Simple SummaryCOVID-19 has had both physical and mental health effects stemming, in particular, from lockdowns. Furthermore, these physical and mental effects have impacted not only people’s lives but also the lives of their pets, which in recent years have often been seen as members of the family. This study aimed to investigate whether the COVID-19 pandemic affected pets and their owners in Japan. Therefore, we conducted an online questionnaire survey with cat and dog owners immediately after the state of emergency was announced in Japan. It comprised questions about owners’ physical activity and mental health, changes in their working conditions, frequency of interaction with pets, and pet behavioural tendencies before and after the COVID-19 lockdown, and then explored the relationships between them. Results showed that both cat and dog owners reported that the behavioural restrictions caused by the COVID-19 lockdown increased the amount of time they spent at home, thus increasing the frequency of contact interaction with their pets. Furthermore, we found that stress-related behaviours were higher in the pets of owners with increased contact interaction. The results of this study indicated that the behavioural restrictions of COVID-19 may have affected the lives of pets, their owners, and their relationships. The results obtained in this study provide important insights into the coexistence of humans and companion animals.AbstractPhysical and mental effects stemming from COVID-19 have impacted not only people’s lives but also the lives of their pets, which in recent years are often seen as members of the family. This study aimed to explore the impact of the COVID-19 pandemic in Japan on pets and their owners. Participants reported changes in physical activity and mental health, as well as working conditions and frequency of interaction with pets, before and after behavioural restrictions due to the COVID-19 pandemic. We also asked about their pets’ behaviours using the Feline Behavioural Assessment and Research Questionnaire (Fe-BARQ) and the Canine Behavioural Assessment and Research Questionnaire (C-BARQ). This study showed that most cat and dog owners spent more time at home due to the COVID-19 behavioural restrictions and that the frequency of contact interaction with their pets increased. However, this study showed higher stress-related behaviours (e.g., cats: excessive grooming; dogs: aggression towards owners) among pets whose owners increased contact interaction. Furthermore, owners’ low mental health was correlated with high stress-related behaviours (e.g., touch sensitivity) in pets. The results of this study indicate that the lockdown caused by the COVID-19 pandemic in Japan may have affected not only the lives of owners but also the interaction between owners and their pets, and consequently their pets’ behaviours. Therefore, there is also concern that changes in lifestyle patterns caused by pandemics could form a negative feedback loop between the health status of both owners and their pets.