The approval, announced on October 6, clears the way for Saudi Arabia to purchase 44 Terminal High Altitude Area Defense launchers and 360 interceptor missiles, as well as fire control stations, radars and trucks.
"We confirm that an agreement was reached with the Kingdom of Saudi Arabia on the supplies of S-400 air defense systems, Kornet-EM anti-tank guided missiles, TOS-1A systems, AGS-30 automatic grenade launchers and AK-103 Kalashnikov rifles", the press service of the Federal Service for Military-Technical Cooperation told TASS on Monday.
Russian President Vladimir Putin hosted Saudi Arabia's King Salman for talks at the Kremlin last week during which Saudi Arabia said it had signed a memorandum of understanding on the purchase of the S-400s.
The department said the sale of the equipment to the Saudi people would help provide a balance to a relatively unstable environment in the Gulf and to help the United States forces enlarge its allied grip on the region.
The THAAD - Terminal High Altitude Area Defence - missile system is used to defend against incoming missile attacks and "is one of the most capable defensive missile batteries in the U.S. arsenal and comes equipped with an advanced radar system", according to AFP.
The following day, the U.S. Defense Security Cooperation Agency announced $15 billion in weapons sales to Riyadh.
Iran, whose hostile behaviour has got the U.S. and Saudi Arabia concerned recently, also happens to have one of the biggest ballistic missile programs in the Middle East.
The United States deployed THAAD to South Korea this year to guard against North Korea's shorter-range missiles.