The arrest is in connection with an incident in Archford Croft, Emerson Valley, Milton Keynes at 11.59pm on Saturday, when officers were called to a report of a stabbing.
A 17-year-old boy died of his injuries at the scene. A second boy, also 17, was taken to hospital where he also sadly died.
The 21-year-old man, from Milton Keynes, has been arrested on suspicion of two counts of murder, and two counts of attempted murder.
He remains in police custody.