The new sanctions, details of which were not released, were to take effect following a 15-day Congressional notification period, she said.
State Department spokeswoman Heather Nauert said it had been determined that Russian Federation "has used chemical or biological weapons in violation of global law, or has used lethal chemical or biological weapons against its own nationals".
NBC, a USA news network, cited a senior state department official as saying that the sanctions would be structured in two tranches.
The Organization for the Prohibition of Chemical Weapons, the global chemical weapons watchdog, concluded in April that the nerve agent Novichok, known to have been first produced by the Soviet military, had been used in the attack against the Skripals, both of whom survived, although it did not assign blame.
In late March, Trump ordered 60 more Russian diplomats expelled from the U.S. as part of a global response to the attack - a response that included similar expulsions of diplomats from other nations checking Russia. One of them died.
The Skripals fell unconscious on a park bench in Salisbury, Wilts, on March 4 after being exposed to a military-grade nerve agent determined to be Novichok.
The US Government said it would impose fresh sanctions on Russia after it determined that Moscow had used a nerve agent against a former Russian agent and his daughter in the UK.
Several members of Congress had expressed concern that the Trump administration was dragging its feet on the determination and had missed a deadline to publish its findings.
Pushed to take action by the Republican lawmaker who chairs the House's Foreign Affairs Committee, the Trump administration moved on Wednesday to take delayed action.
"The mandatory sanctions that follow this determination are key to increasing pressure on Russian Federation". The US still wants to maintain relations with Moscow, despite the new sanctions.
It would reportedly include more drastic measures, such as downgrading diplomatic relations, banning the Russian airline Aeroflot from flying to the USA and cutting off almost all exports and imports.