Successful social interactions rely on flexibly tracking and revising others’ beliefs. These can be revised prospectively, new events leading to new beliefs, or retrospectively, when realizing that an attribution may have been incorrect. However, whether infants are capable of such belief revisions is an open question. We tested whether 18-month-olds can revise an attributed FB into a TB when they learn that a person may have witnessed an event that they initially thought she could not see. Infants first observed Experimenter 1 (E1) hiding two objects into two boxes. Then E1 left the room, and the locations of the objects were swapped. Infants then accompanied Experimenter 2 (E2) to the adjacent room. In the FB-revised-to-TB condition, infants observed E1 peeking into the experimental room through a one-way mirror, whereas in the FB-stays-FB condition, they observed E1 reading a book. After returning to the experimental room E1 requested an object by pointing to one of the boxes. In the FB-stays-FB condition, most infants chose the non-referred box, congruently with the agent’s FB. However, in the FB-revised-to-TB condition, most infants chose the other, referred box. Thus, 18-month-olds revised an already attributed FB after receiving evidence that this attribution might have been wrong.