The new sanctions, details of which were not immediately released, were to take effect following a 15-day Congressional notification period, she said.
The State Department said in a statement that under the 1991 Chemical and Biological Weapons Control and Warfare Elimination Act, Russia was found to have "used chemical or biological weapons in violation of worldwide law or had used lethal chemical or biological weapons against its own nationals".
The new announcement could bolster Mr Trump's claim that his administration is taking a tough stance on Moscow, even as he denounces special counsel's Robert Mueller's Russian Federation probe as a "witch hunt" that ought to be halted immediately.
On March 4, Sergei Skripal, 66, who had been convicted in Russia for spying for the United Kingdom but later swapped for Russian intelligence officers, and his daughter Yulia, 33, were found unconscious on a bench near the Maltings shopping center in Salisbury.
European countries and the United States expelled 100 Russian diplomats after the attack, in the strongest action by President Donald Trump against Russia since he came to office.
Police found a bottle containing the substance at the scene.
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.
"The mandatory sanctions that follow this determination are key to increasing pressure on Russian Federation".
While criticized as too keen to strike up a friendship with Putin, Trump maintains that he's been tough on Moscow.
Russian officials question why Russia would want to attack an ageing turncoat who was pardoned and swapped in a Kremlin-approved 2010 spy swap.
The State Department announced a number of possible exceptions to the sanctions announced Wednesday.