Group-based settings are valuable for promoting physical activity when individuals perceive high groupness, or when they feel like they are exercising in a true group (Spink, Wilson, & Priebe, 2010). Guided by basic needs theory , we tested a mechanistic model that examined the extent that groupness perceptions within fitness classes predicted satisfaction of basic needs (i.e., competence, relatedness, and autonomy) and, in turn, basic needs predicted general satisfaction as well as weekly group fitness bouts. Exercisers in fitness facilities in the United States and New Zealand (N ϭ 293, M age ϭ 35.93, SD ϭ 11.44, 78% female, 22% male) completed a survey directly following a class entailing groupness subscales of Entitativity (e.g., perceiving the collective as a group) and Group Structure (e.g., reporting group norms), followed by an online survey 3 weeks afterward. Multiple mediation models revealed an indirect effect of groupness subscales through basic need satisfaction, on both general satisfaction and weekly fitness bouts. Autonomy and relatedness were the strongest mediators when predicting general satisfaction, whereas relatedness was the strongest mediator when predicting weekly fitness bouts. These findings support the use of basic needs theory for understanding how exercisers are influenced by their groups. Practically, groupness-enhancing strategies have the potential to impact belongingness along with competence and autonomy perceptions as critical evaluations for promoting health behavior.