During the COVID-19 pandemic, interpersonal relations were highly constrained due to the social distancing rules and sanitary restrictions imposed to prevent the transmission of the virus. These social changes gave rise to ever-deepening experiences of loneliness, deterioration of mental well-being, and fear of COVID-19, which affected the citizens of all the countries struggling with the pandemic. The present paper reports the results of a study on mental health, interdependent happiness, loneliness and fear of COVID-19 in Polish and Japanese university students. A total of 180 people (83 Poles and 97 Japanese) aged 19-41 participated in the study. The participants were surveyed using the Mental Health Continuum-Short Form to assess three dimensions of mental health; the UCLA Three-Item Loneliness Scale to measure loneliness; the Interdependent Happiness Scale based on the concept of a relational sense of community with others as associated with well-being, and the Fear of COVID-19 Scale (FCV-19S). The respondents were also asked one question about their subjectively perceived economic status. The largest cross-cultural difference in the investigated variables concerned fear of COVID-19. The Japanese university students showed higher levels of this fear. The level of fear of COVID-19 in the group of women, regardless of the culture they came from, was not associated with any other variables we analyzed. In the Japanese sample, it was negatively correlated with interdependent happiness, and in the Polish sample, fear od COVID-19 was only correlated with the Social dimension of Mental Health. Other variables that differentiated the two nationalities, though to a lesser extent, were the Social dimension of Mental Health, which was higher in the Japanese sample, and the Psychological dimension of Social Health, with higher scores in the Polish sample. The power of mental resources, although it may vary among individuals, is independent of culture. Regardless of the level of fear of COVID-19 in the two cultures, the examined mental resources are positively associated with well-being and the return to normal functioning after the pandemic.