Witness video shows flames shooting through the roof of the home.
The house on Mary Street near Belmont Avenue caught fire at around 2 a.m. Sunday.
Action News is told a man was trapped in a first-floor apartment.
Elsmere police officers rushed in to help rescue him.
The man is critically injured, being treated for burns and smoke inhalation.
The officer's condition has not been released.
Two people living in a second-floor apartment escaped unharmed.
The cause of the fire is still being investigated.