A student graduation truck was parked near the water on the island of Djurgården on Thursday afternoon. The students, who had been drinking aboard the truck, decided to go for a swim. Nobody noticed initially when one man went missing.
Police were alarmed at 2:13pm and blocked off the area. Divers were able to locate the student after 20 minutes.
"He was taken to Karolinska Hospital, where it was determined his life could not be saved," said Stockholm police Albin Näverberg.
The incident marked the latest in a string of accidents connected to the high school celebrations.
Earlier in the week, a student in Dalarna fell off one of the trucks and was run over. Last week, a similar parade was cancelled in central Sweden after two students fell off one of the trucks. Police on the scene said the students' drunken behaviour was "worse than ever before".