The murder took place exactly five years, to the day, after the disappearance of American teen Natalee Holloway in Aruba.
Van der Sloot remains the main suspect in that case.
In court today in Peru, he said he is "truly sorry" for the killing of Stephany Flores. He said he had "wanted from the first moment to confess sincerely" to the killing. His lawyer told the court that van der Sloot killed Flores as a result of "extreme psychological trauma" he had suffered from the fallout over the Holloway case.
Van der Sloot faces a possible 30 years in prison in the killing, though the guilty plea was aimed at winning a lighter sentence. The defense had claimed the killing was manslaughter, which carries a minimum sentence of five years.
In a confession, he had said he killed her in a fit of rage after she discovered on his laptop that he'd been linked to the Holloway disappearance.
But prosecutors said he killed her to rob her after learning she had won money at the casino where they met.