Physical activity (PA) is a major factor related to obesity risk. Research has shown PA interventions among adolescents to be moderately successful in short-term but limited to longer-term. Self-determination theory (SDT) postulates that a psychological need-supportive environment (i.e. one that supports competence, autonomy, and social relatedness) is effective in maintaining volitional motivation which can lead to sustained positive behavioral changes including PA. Although research has supported the central tenets of the SDT, there is limited evidence examining whether a summer camp intervention can sustain improvements in PA motivation and behavior. Thus, this study examined the acute and 12-weeks longer-term effectiveness of a five-day psychological need-support centered summer camp on healthy weight and overweight adolescent girls’ weight management behaviors. A single-group case series study with pre-, post, and 12-week follow-up-test analyses. A sample comprised 42 (Mage = 11.70±1.12) adolescent females. Exercise motivation, PA intention, and PA and dietary behaviors were measured. The findings showed a between-group effect on daily steps (F(1, 19) = 15.83, p = .001, ?p2 = .46), moderate-to-vigorous intensity PA (F(1, 19) = 4.58, p = .046, ?p2 = .19), energy intake (F(1, 19) = 7.23, p = .013, ?p2 = .27), PA intention (F(2, 18) = 6.25, p = .024, ?p2 = .28), intrinsic motivation (F(2, 18) = 6.25, p = .024, ?p2 = .28), and amotivation (F(2, 18) = 16.25, p < .001, ?p2 = .54). A need-supportive summer camp may be especially effective in improving PA motivation and behavior in overweight girls.