"This has resulted in a failure of the heart, which is indicated as the suspected cause of death," they said.
Praljak drank from a vial after his 20-year term for war crimes in Bosnia in the 1990s had been upheld on Wednesday.
Dutch authorities are investigating how he smuggled poison into the court.
The confirmation rulings against Praljak and five other defendants brought an end to more than 20 years of work by the International Criminal Tribunal for the former Yugoslavia (ICTY).
What happened in the courtroom on Wednesday?