It happened at a home near Glenrosa and 55th Avenues in Phoenix.
First responders say it's unclear how the girl got into the pool because the ladder was turned away from the outside.
"This pool style is very, very common," said Aaron Ernsberger with the Phoenix Fire Department. "It's an above ground pool and they do possess a lot of safety features to them, however they are still accessible by young kids, and as you can see this family and this community is going to be dealing with this problem right now."
An adult performed CPR on the girl until firefighters arrived. Fox 10 will bring you updates on the toddler's condition as more details become available.