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 USA forces enlarge its allied grip on the region.
The transaction covers the delivery of 44 Terminal High Altitude Area Defense launchers, 360 interceptor missiles, 16 fire control and communications mobile tactical station groups and seven AN/TPY-2 radars.
He noted that the faith that the Russian leadership has in the kingdom led it to request that Saudi Arabia become its partner in this strategic military deal.
Saudi Arabia is a semi-continent that needs to protect is land, air and marine borders through possessing air defense capabilities and other weapons that will enable it to lead tactical military work, he explained.
Meanwhile, Kremlin spokesman Dmitry Peskov told reporters Monday that talks with Riyadh about buying its S-400 air-defense system were going well despite Saudi Arabia's THAAD order with the U.S. It is the first visit of a Saudi monarch to visit Mr Putin.
The US sale has not yet "concluded", it confirmed. It can carry three types of warheads created to destroy targets including aircraft, as well as ballistic and cruise missiles.
The sale of the anti-aircraft systems to USA allies - Turkey and Saudi Arabia - was criticized in Washington, but did not affect the outcome of the deals.
Iran, whose hostile behaviour has got the United States and Saudi Arabia concerned recently, also happens to have one of the biggest ballistic missile programs in the Middle East.
This sale is likely to further U.S. national security and foreign policy interests and support the long-term security of Saudi Arabia and the Gulf region.
Saudi Arabia would not let anyone finance terrorism and spread the ideology of hatred, Saudi Foreign Minister Adel al-Jubeir said in a news conference for Russian media.
Both Washington and Saudi Arabia accuse Tehran of aggressive and disruptive behavior in the region.
Its recent deployment by the USA military in South Korea to protect against any North Korean strike drew protests from Beijing, who feared its sensors would be able to penetrate into Chinese air space and upset the balance of power. It is also the recognition that Russian Federation is an important mediator in Saudi Arabia's relations, for example with Iran.