The 44-year-old man suffered non life-threatening injuries, Ottawa city police say.
It's not clear what sparked the incident and police have not released the victim's name.
A man was taken into custody at the scene with help from RCMP officers who were near the building in the Sandy Hill neighbourhood at the time of the stabbing, shortly after 8:30 a.m. EDT.
No charges have been laid, but that could change, said Ottawa police Insp. Murray Knowles.
The man in custody is a Canadian, said Knowles, who declined to release his identity.
The embassy was expected to release a statement later.