0556 GMT November 13, 2019
President Recep Tayyip Erdogan said Ankara had already paid the deposit on the deal, estimated to be worth $2.5 billion (£1.9 billion), BBC reported.
Turkey has the second-largest army in NATO. Alliance members are urged to buy compatible weapons systems.
Turkey has been establishing closer links with Russia after its recent souring of ties with the US.
Turkey objects to US military support for the YPG Syrian Kurdish rebels, who are linked to rebel Kurds in Turkey.
Russia says the S-400 system has a range of 400km (248 miles) and can shoot down up to 80 targets simultaneously, aiming two missiles at each one.
Russia deployed the S-400 at its air force base near Latakia in Syria in December 2015, after Turkish jets had shot down a Russian Su-24 warplane on the Syria-Turkey border.
That incident caused a diplomatic rift between Russia and Turkey, but President Erdogan later patched up his quarrel with President Vladimir Putin.
A military adviser to Putin, Vladimir Kozhin, said the S-400 contract with Turkey was "strictly compatible with our strategic interests". "On that score, one can quite understand the reaction of some Western countries who are trying to put pressure on Turkey."
Erdogan, quoted by Turkey's Hurriyet daily, voiced displeasure with unnamed Western partners who were "seeking enormous amounts of money" for military drones.
He said Turkey had killed 90 YPG "terrorists" in the past week with Turkish drones — developed because the Western ones were too expensive.