Background: Obesity is a major public health concern worldwide. A sedentary life and a nutritional transition to processed foods and high-calorie diets are contributing factors to obesity. The demand for nutraceutical foods, such as herbal weight-loss products, which offer the potential to counteract obesity, has consequently increased. We hypothesised that Opuntia cladodes consumption could assist weight management in an obesity prevention context. Methods: This study was designed to explore the anti-adipogenic effects of lyophilised Opuntia cladode powders (OCP) in an in vitro cellular model for adipocyte differentiation and an in vivo high-fat-diet (HFD)-induced obesity rat model. Two OCP were tested, one from wild species O. streptacantha and the second from the most known species O. ficus-indica. Results: Pre-adipocytes 3 T3-F442A were treated by OCP during the differentiation process by insulin. OCP treatment impaired the differentiation in adipocytes, as supported by the decreased triglyceride content and a low glucose uptake, which remained comparable to that observed in undifferentiated controls, suggesting that an antiadipogenic effect was exerted by OCP. Sprague–Dawley rats were fed with a normal or HFD, supplemented or not with OCP for 8 weeks. OCP treatment slightly reduced body weight gain, liver and abdominal fat weights, improved some obesity-related metabolic parameters and increased triglyceride excretion in the faeces. Taken together, these results showed that OCP might contribute to reduce adipogenesis and fat storage in a HFD context, notably by promoting the faecal excretion of fats. Conclusions: Opuntia cladodes may be used as a dietary supplement or potential therapeutic agent in diet-based therapies for weight management to prevent obesity.