We investigate a novel approach to newcomer socialization based on self-determination theory (SDT). A core assumption of SDT is that when social contexts support basic psychological needs for autonomy, competence, and relatedness, people are more likely to integrate new experience effectively and thrive in their environment. We sought to examine the role of psychological need support within the context of newcomer socialization and the period of early entry where newcomers learn their new role and become integrated within the organization. We propose that organizational socialization tactics and perceived autonomy-supportive supervision jointly influence newcomers’ basic psychological needs and, in turn, their organizational commitment and withdrawal cognitions. Results from structural equation modeling analyses from a time-lagged study of 489 MBA interns supported our hypothesized model. There were significant indirect effects of institutionalized socialization tactics and supervisor autonomy support on both affective organizational commitment and withdrawal cognitions, via psychological need satisfaction. Use of institutionalized tactics also was negatively associated with interns’ specific need for autonomy, suggesting that individualized tactics may play a role in supporting newcomers’ sense of self-determination. A post hoc moderation analysis further suggested a substitutive pattern in the interaction between supervisor autonomy support and institutionalized tactics, emphasizing the central role that supervisors play in newcomer socialization, particularly when it pertains to newcomers’ psychological need satisfaction. Our results indicate that SDT is a promising theoretical framework for studying newcomer adjustment.