There is a paucity of evidence on the heterogeneous impacts of financial incentives on weight loss. Between March 2010 and January 2012, in a randomized controlled trial, we assigned 700 obese persons to three experimental arms. We test whether particular subgroups react differently to financial incentives for weight loss. Two treatment groups obtained a cash reward (€150 and €300 with 237 and 229 participants, respectively) for achieving an individually-assigned target weight within four months; the control group (234 participants) was not incentivized. Participants and administrators were not blinded to the intervention. We find that monetary rewards effectively induced obese individuals to reduce weight across all subgroups. However, there is no evidence for treatment-effect heterogeneity for those groups that were incentivized. Among those who were in the €300 group, statistically significant and large weight losses were observed for women, singles, and those who are not working (all above 4 kg in four months). In addition, the magnitude of the reward matters only for women and migrants. The effectiveness of financial incentives to reduce weight nevertheless raises sensitive ethical issues that should be taken into consideration by policymakers.