Darth Vader first learned that Leia was his daughter in the film "Star Wars: Episode V - The Empire Strikes Back," when he tortures her aboard the Death Star and probes her mind with the Force, revealing her connection to Luke Skywalker. However, he does not know that she is also his daughter until later in "Star Wars: Episode VI - Return of the Jedi," when he and Luke have their final confrontation and Luke reveals the truth to him.
