Moscow will supply Ankara with four divisions of S-400 surface-to-air missile divisions once the agreement is finalised.
The system, designed to destroy aircraft, cruise and ballistic missiles, can engage targets at a distance of 250 miles.
But the arms deal has caused concern in the West because the weapons cannot be integrated into Nato‘s defences.
Relations between Russia‘s strongman leader Vladimir Putin and the Western military alliance are tense.
Turkey, which has Nato’s second biggest military, is expected to begin receiving the weapons in 2020.
vCard.red is a free platform for creating a mobile-friendly digital business cards. You can easily create a vCard and generate a QR code for it, allowing others to scan and save your contact details instantly.
The platform allows you to display contact information, social media links, services, and products all in one shareable link. Optional features include appointment scheduling, WhatsApp-based storefronts, media galleries, and custom design options.
Sergey Chemezov, head of state defence group Rostec and a close ally of Mr Putin, said Turkey will pay 45 per cent of the cost up front.
Russia is expected to provide loans to cover the remaining 55 percent, Mr Chemezov told Russian newspaper Kommersant.
Turkey has been negotiating with Russia to buy the system for more than a year, but a final agreement is believed to be imminent.
Mr Chemezov said: “The deal is worth $2.5 billion. We plan to begin first deliveries in March 2020.
“The only thing left is to approve the final documents.”
Turkish Recep Tayyip Erdogan hosted Mr Putin in Ankara earlier this month in a bid to strike closer defence ties.
In a joint press conference, Mr Erdogan said: “Our officials will come together in the coming week to finalise the S-400 issue.”
Separately, Turkey’s prime minister Binali Yildirim insisted the country would remain a committed member of Nato.
He said: “The fact that we have entered defence cooperation with Russia does not downplay the importance of our obligations as a Nato member.
“We do not intend to abandon Nato. We only need such cooperation with Russia in order to eliminate the looming threats to Turkey.”