Why individuals believe in conspiracy theories is a theoretical question with practical implications. Research has mostly focused on individual differences and motives, and we therefore know little about how manipulated variables may affect conspiracism. Based on the truth effect literature, which has demonstrated that statements already seen (or perceived as already seen) are more likely to be judged as true than new ones, we hypothesized that repeated exposure to conspiracy theories can increase the likelihood of their being believed. If this were confirmed, it would help to challenge truth ambiguity as a boundary condition of the truth effect. As an initial test of this hypothesis, we analyzed data from two surveys conducted in representative samples of the French population (IFOP, 2017, 2019). Participants indicated both their adherence to and recognition of conspiracy statements in each survey, corresponding to a total of 17 widespread conspiracy theories. We found a truth effect in each dataset, although with different magnitudes. This truth effect was positively associated with a conspiracy mentality. These results suggest that individuals may form beliefs in conspiracy theories through the truth effect, and lend further weight to the reassessment of truth ambiguity as a boundary condition of the truth effect.