This study investigated mediating and moderating factors on the relationship between newcomers’ proactive personality and job satisfaction. 203 newcomers participated in the survey 6 months after the entry. An additional survey has been conducted right after the entry in order to measure the change in the person-organization fits. Results showed that newcomers’ proactive personality has a positive relation with their job satisfaction. This relationship, however, was moderated by socialization tactics such that it is significant only when the organization employs institutionalized socialization tactics rather than individualized socialization tactics. Finally, newcomers’ proactive personality affected job satisfaction being fully mediated by the change of person-organization fit. These results imply that an individual’s proactive personality can influence the adaptation process to a new organization. It provides practical implications for organizations to effectively design and implement the selection and socialization practices.