The objective of this systematic review was to appraise current knowledge on the impact of physical activity (PA) and physical fitness (PF) on the health of class II and III obese subjects and bariatric surgery (BS) patients. All original studies were searched using four databases (Medline®, Scopus®, CINAHL and Sportdiscus). Two independent investigators selected studies assessing the impact of PA or PF on specific health outcomes (anthropometric parameters, body composition, cardiometabolic risk factors, PF, wellness) in adults with a body mass index ≥35 kg m−2 or in BS patients. Conclusions were drawn based on a rating system of evidence. From 3,170 papers identified, 40 papers met the inclusion criteria. The vast majority of studies were recently carried out with a predominance of women. Less than one-third of these studies were experimental and only three of them were of high quality. Each study reported at least one beneficial effect of PA or PF. However, a lack of high-quality studies and heterogeneity in designs prevented us from finding high levels of evidence. In conclusion, although results support the importance of PA and PF to improve the health of this population, higher-quality trials are required to strengthen evidence-based recommendations.