The woman, also 21, suffered fatal injuries in Riverside Park in Aberfeldie, northwest of the CBD, about 11.30pm (AEST) on Thursday.
Paramedics could not revive the woman and she died at the scene.
A 21-year-old Essendon man was then arrested in the park, interviewed and charged with murder.
Police say the suspect and the slain woman were known to each other.
‘We believe that there was a relationship, possibly, but again we’re still working through the details,’ Homicide Detective Sergeant Mark Hatt told reporters at the scene.
The 21-year-old man will face the Melbourne Magistrates Court on Friday.