The deal comes amid between worsening relations between Turkey and its NATO partners.
Turkey has agreed to purchase the S-400 surface-to-air missile-defense system.
President Recep Tayyip Erdogan said he signed a deal to buy the missile system on Tuesday, according to The New York Times, citing local media.
"Signatures have been made for the purchase of S-400s from Russia," Erdogan said. "A deposit has also been paid as far as I know."
The prospective purchase was first reported several months ago, but Erdogan's announcement is the first confirmation Ankara has given that it has paid for the weapons system.
Turkey's membership in NATO would typically require purchasing weapons systems approved by the defense bloc, but the deal comes amid worsening tensions between Turkey and its NATO partners.
Turkey started looking for its own missile-defense system after the US, Germany, and the Netherlands, declined to renew their Patriot-missile deployments in southern Turkey.
The S-400 system can detect and target missiles, manned and unmanned aircraft, and hit targets up to 250 miles away. But it is not compatible with NATO systems, nor would it be subject to the same NATO limits on deployment, meaning that Ankara could set it up in places like the Armenian border or Aegean coast.
The S-400 deal "is a clear sign that Turkey is disappointed in the US and Europe," Konstantin Makienko, an analyst at Moscow-based think tank the Center for Analysis of Strategies and Technologies, told Bloomberg in July.
Though Turkey is a longtime NATO member, its ascension to the EU looks increasingly unlikely, and Erdogan has accused the regional bloc of "messing us about" over issues like visa agreements and Syrian migrants. The Turkish government was also frustrated by NATO's response to the attempted coup against Erdogan in summer 2016.
"Nobody has the right to discuss the Turkish republic’s independence principles or independent decisions about its defense industry," Erdogan said on Tuesday — a remark that may have been directed at NATO.
"We make the decisions about our own independence ourselves — we are obliged to take safety and security measures in order to defend our country."
The deal with Russia may also help Turkey kick-start its domestic defense industry — a goal that may be related to Western reticence to share advanced technology.
Relations between the US and Turkey in particular have been strained in recent months, after a May incident in which bodyguards traveling with Erdogan kicked and punched peaceful protesters outside the Turkish ambassador's residence in Washington.
Nineteen people — 15 of them Turkish security officials — have been indicted in relation to the incident, and a bill introduced to the Senate this week would block the US from supporting further arms sales to forces protecting the Turkish president.