From 19 December 1999 to December 2003, Kosachev was a deputy of the 3rd State Duma of the Russian Federation, the first deputy head of the Fatherland – All Russia political faction, and deputy chairman of the Foreign Affairs Committee. Kosachev was a member of the Commissions to assist the Federal Republic of Yugoslavia in overcoming the consequences of the NATO bombing, to implement the Anti-Ballistic Missile Treaty between the Russian Federation, the Comprehensive Nuclear-Test-Ban Treaty (CTBT) and to conclude a new treaty on further reduction and limitation of strategic offensive arms (New START).