Background: With the worldwide shortage of organs to meet the demand for transplants, many countries are considering whether introducing opt-out consent would increase the availability of donor organs. This research assessed whether people’s willingness to donate their organs for transplant purposes was greater in opt-out than opt-in countries and whether such effects were moderated by people’s awareness of their nation’s legislative system. Methods: Using secondary data from the 2010 Eurobarometer (n=29,288), this study compared people’s willingness to donate their organs in a representative sample from 19 opt-out and 10 opt-in consent countries from across Europe. Results: The majority of participants (66.04%) stated that they were willing to donate their organs for transplant purposes. The proportion of people who were willing to donate their organs did not differ between opt-in (65.97%) and opt-out (66.37%) countries. However, the effect of consent on willingness to donate was moderated by people’s awareness of their nation’s organ donation legislation. When people were aware of their nation’s legislation, the proportion of people who were willing to donate their organs was greater in opt-out (85.26%) than opt-in (80.72%) countries. By contrast, when people were not aware of their nation’s legislation, there was no difference in people’s willingness to donate their organs in opt-in (58.63%) and opt-out (59.23%) countries. Conclusions: Opt-out consent countries should increase people’s awareness of their legislation to improve donor rates. Further research is needed to understand the factors that moderate the effect of opt-out consent legislation on organ donation.