Police say at about 11:30 Saturday night, 30-year-old Aaden Brindamour climbed a fire escape to get into his ex-girlfriend's apartment.
Police say he then stabbed her new boyfriend several times.
The girlfriend managed to escape unharmed.
Police evacuated the building, and a standoff ensued.
Five hours later, officers persuaded the man to surrender.
The stabbing victim underwent surgery for wounds to his neck and arm, and is listed in serious, but stable condition.