Although there was no public signing, the deal was sealed during President Vladimir Putin's ongoing visit to New Delhi for an annual summit with Indian Prime Minister Narendra Modi.
"The deal was signed on the fringes of the summit," Kremlin spokesman Dmitry Peskov told Reuters. The contract is estimated to be worth more than $5 billion and gives the Indian military the ability to shoot down aircraft and missiles at unprecedented ranges.
The United States has said countries trading with Russia's defense and intelligence sectors would face automatic sanctions under a sweeping legislation called Countering America's Adversaries Through Sanctions Act (CAATSA).
A State Department spokesperson said this week that the implementation of the sanctions act would be focused at countries acquiring weapons such as the S-400 missile batteries.