It follows last weekend's failed military coup, with tens of thousands arrested, suspended or dismissed in the fallout.
President Erdogan also says he'll support the return of the death penalty if it's approved by the national parliament.
It was an announcement delivered as the local clock approached midnight.
Turkish citizens and pro-government supporters again packed the streets of Ankara and Istanbul to watch the President's address on television.
Recep Tayyip Erdogan declared a three-month state of emergency, exercised under article 120 of Turkey's constitution.
He says it is to eradicate any remaining support for coup conspirators, after last weekend's chaos.
"To be able to take the most efficient steps in order to remove this threat as soon as possible, which is a threat to democracy, to the rule of law, to the rights and freedoms of our citizens in our country. This practice is absolutely not against democracy, the rule of law, and freedoms. Quite on the contrary, it has the purpose of strengthening and protecting these values."
A state of emergency could mean sweeping powers of arrest and detention, press censorship, and curfews.
Some 60,000 soldiers, police, judges, public servants and teachers have been suspended, detained or are under investigation.
Academics have been banned from international travel, and those abroad asked to return home.
Opposition politician Ozgur Ozul has told the BBC the aftermath shouldn't be exploited for political gain.
"We have witnessed many coups in the past and have stopped this one altogether. But no one has the right to exploit our good will, and we will continue to support democracy and everyone should do the same. No one should turn this into an opportunity because we will resist him the same way we fought the coup plotters."
Turkey is proceeding with an extradition request for United States-based cleric Fetullah Gulen, who the government blames for orchestrating the coup attempt.
President Erdogan told Al Jazeera the battle against Mr Gulen and his supporters is not over.
"Fetullah terrorist organisation had a parallel organisation within the state and every step they have taken in making the attempt and failed coup, I don't think we have come to the end of it yet. And in the days and weeks ahead, I am sure, they have some of their own ideas and plans on how to go about their own future."
Mr Gulen denies any involvement, but says he will respect the extradition order if the US issues one.
Arrests and the sacking of public servants is continuing, but there has been minimal public outcry.
Many say they want the coup leaders hanged and Turkey united.
"We want a beautiful Turkey, we don't want it like Syria or Gaza. We want peace in Turkey. They will never be able to split us up. We will never allow it."
