Nikolas Cruz, 23, also pleaded guilty to 17 counts of attempted murder for those he injured in the attack on Marjory Stoneman Douglas High School.
He faces the possibility of the death penalty or life in prison.
One of the deadliest school shootings in US history, the incident became a rallying cry for gun control activists.
Mr Cruz was 19-years-old when he shot dead 14 students and three employees with an AR-15 rifle at his former school. Another 17 people were wounded.
