Recep Tayyip Erdogan, Turkey's president, announced on Tuesday that "signatures have been made" for the acquisition of the S-400 surface-to-air missile defence equipment.
Anticipating potential concern among Turkey's fellow NATO members over the deal, Erdogan said Turkey would continue to "make the decisions about our own independence ourselves".
"We are obliged to take safety and security measures in order to defend our country … Nobody has the right to discuss the Turkish republic's independence principles or independent decisions about its defence industry," he said.
Russia confirmed the agreement, with Vladimir Kozhin, Russian President Vladimir Putin's adviser for military cooperation, saying: "The contract has been signed and is being prepared for implementation."
Kozhin told Russia's state-owned TASS news agency that all aspects of the deal "strictly comply with our strategic interests".
"For this reason, we fully understand the reactions of several Western countries which are trying to put pressure on Turkey," he said.
The deal is expected to cause anxiety for NATO's 28 other member countries, as the Russian-made equipment may not prove technically compatible with defence systems operated by the alliance.
A Pentagon spokesman criticised the procurement decision, saying that, "generally it's a good idea" for members of NATO to buy inter-operable equipment.
NATO's policy states that: "Interoperability does not necessarily require common military equipment", but instead, "what is important is that the equipment can share common facilities, and is able to interact, connect and communicate, exchange data and services with other equipment".
The Turkish-Russian contract is a new sign of better relations between the two countries since a reconciliation deal was signed last year following the 2015 shooting down by the Turkish military of a Russian fighter jet over the Syrian border.
Turkey was also pleased with Russia's response to the 2016 failed coup in Turkey, and the two sides have been working together in search of a solution to the Syria conflict.