Emergency services were called to Stonehaven at about 7pm on Wednesday following reports of an incident in the High Street area.
A 21-year-old man was taken to Aberdeen Royal Infirmary to be treated for serious injuries.
A 28-year-old was arrested in connection with the incident, Police Scotland said.
Detective Inspector Stewart Drummond said: “This has been a contained incident and I would like to thank members of the public for their co-operation and patience while inquiries were progressed in the area.
“Inquiries remain ongoing and anyone with information about this incident or who was in the area at the time is asked to contact Police Scotland on 101.”