Background: Considerable numbers of young people are not meeting physical activity guidelines. Wearable fitness devices can provide opportunities for physical activity promotion. Purpose: The aim of the study was to explore whether wearable healthy lifestyle technologies impacted on adolescents' (13- to 14-year-olds) motivation for physical activity. Methods: The study was a mixed method sequential design. Participants were 84 adolescents (44 girls, 40 boys) from 6 physical education classes. Pupils were issued with a Fitbit to wear for 8 weeks and completed pre-/posttest questionnaires that assessed motivational regulation and psychological need satisfaction. Adolescents also engaged in focus group interviews after wearing the Fitbit for 8 weeks. Quantitative data were analyzed using a repeated measures multivariate analysis of variance (MANOVA) to explore differences between gender and time. Qualitative data analysis was conducted deductively using self-determination theory. Results: The quantitative findings identified significant reductions in need satisfaction and autonomous motivation and significant increases in amotivation after 8 weeks. Qualitative evidence suggested short-term increases in motivation through feelings of competition, guilt, and internal pressure. Discussion: Findings suggest that healthy lifestyle technology may have negative motivational consequences. Translation to Health Education Practice: Certified Health Education Specialists should support young people to personalize health targets in order to critically engage with normalized health targets.