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.
The case will now head to a penalty trial in which jurors must determine whether Mr Cruz is spared the death penalty to face life without parole.
Judge Elizabeth Scherer has said she hopes that the case - for which thousands of jurors will have to be screened - can begin in January.
In court on Wednesday, Judge Scherer asked Mr Cruz how he pleaded to each murder.
Following the plea, Mr Cruz tearfully addressed the judge and the victims' families.
"I am very sorry for what I did and have to live with it every day," he said. "If I were to get a second chance, I would do everything in my power to help others."