The 56-second video from surveillance cameras shows the boy riding his bike Tuesday when the dog attacks him. The cat immediately charges out and scares off the much-larger dog.
The boy suffered bite wounds, but his family says he's going to be OK.
"My cat defends my son during a vicious dog attack and runs the dog off before he can do additional damage. Thankfully, my son is fine!" uploader Roger Triantafilo wrote with the YouTube video.