Background: The emergence of the COVID-19 has obstructed people from participating in various activities. University students were unable to take classes in person and to be involved in school clubs or voluntary activities. Eventually, it raised their sedentary behavior and physical inactivity during this critical period of habit formation. Purpose: The present study aimed to compare beliefs influencing physical activity (PA) intention and behavior before and after the COVID-19 outbreak based on the theory of planned behavior. Methods: Data were gathered three times from Korean undergraduate students. Participants’ beliefs about PA were elicited by think-aloud interview using an open-ended questionnaire. Their answers were analyzed by content analysis and used to make a closed-ended questionnaire for the second survey. Finally, data were collected one month after the second survey for a follow-up survey on PA participation. To understand the influence of salient PA beliefs on PA intention and behavior, data were analyzed using structural equation modeling. Results: There have been quite changes in salient beliefs of PA after emergence of COVID-19. Moreover, despite participation in PA was reduced by half (t = -5.70, p < 0.01), intention to PA has been significantly increased (t = 3.13, p < 0.01). The results also showed that the salient beliefs influencing PA intention and behavior after emergence of COVID-19 were completely different from those before COVID-19. Conclusions: It is estimated that the emergence of COVID-19 has brought changes in PA behavior by transforming various psychological mechanisms.