The footage shows the officer arriving at the scene as the man in a bloodied shirt is bent over a woman on the sidewalk.
The officer orders him multiple times to "drop the knife, drop the knife."
Instead, he stands up and begins advancing toward the officer, knife still in hand. The officer can be seen on the video shooting twice as he starts running toward her.
Officials said the man, identified as William Fuller, 30, later died at the hospital.
Las Vegas police are praising the actions of Sgt. Daniella Cino, saying she acted "bravely and decisively" and saved the life of the woman who had been stabbed.
They noted young children were also present at the time the attack occurred.