PM Theresa May said the diplomats would be expelled after Moscow refused to explain how a Russian-made nerve agent was used on a former spy in the UK.
Moscow continues to deny any involvement in the attack.
US President Trump’s spokeswoman accused Russia of undermining the security of countries worldwide.
BBC’s North American editor Jon Sopel said the White House statement was notable in the unqualified support it offers Theresa May.
He said it was also significant because of the way President Trump was prepared to talk about Russia – using language not been heard from the White House before.
In the statement, Mr Trump’s press secretary Sarah Sanders said the US wanted to ensure “this kind of abhorrent attack does not happen again”.
She described the expulsion of Russian diplomats from Britain as “a just response”.
“This latest action by Russia fits into a pattern of behaviour in which Russia disregards the international rules-based order, undermines the sovereignty and security of countries worldwide, and attempts to subvert and discredit Western democratic institutions and processes,” she said.
Former spy Sergei Skripal, 66, and his daughter, Yulia Skripal, 33, remain critically ill in hospital after being found slumped on a bench in Salisbury, Wiltshire, on 4 March.
Mrs May said the chemical used in the attack had been identified as being part of a group of nerve agents developed by Russia known as Novichok.
She said there was “no alternative conclusion” than to believe Russia was “culpable” for the poisonings.
The White House statement echoed earlier comments made by the US ambassador to the UN, Nikki Haley, who cited the “special relationship” between the two countries and said the US would “always be there” for the UK.
Also addressing the UN Security Council, Britain’s deputy UN ambassador, Jonathan Allen accused Russia of breaking its obligations under the Convention on the Prohibition of Chemical Weapons.
In response, the Russian ambassador to the UN, Vasily Nebenzya, denied Moscow’s involvement in the attack and demanded “material proof” from Britain to support its charge.
Russian foreign minister Sergei Lavrov said Moscow would co-operate in the case if it received a formal request for clarification from the UK under the Chemical Weapons Convention, which sets a 10-day time limit for a response.
Moscow refused to meet the UK’s deadline to co-operate, prompting Mrs May to announce the diplomats’ expulsion and other measures intended to send a “clear message” to Russia.
In a statement to MPs Mrs May said that Russia had provided “no explanation” as to how the nerve agent came to be used in the UK, describing Moscow’s response as one of “sarcasm, contempt and defiance”.
The expulsion of the diplomats, who have been given one week to leave the country, is the largest since 31 were ordered out in 1985 after double agent Oleg Gordievsky defected.
Russia’s foreign ministry said Mrs May’s statement was “an unprecedentedly crude provocation” and that the UK government had “seriously aggravated” relations by announcing a “whole set of hostile measures”.
The Russian embassy in London also tweeted a picture of a thermometer dropping to -23C and said “we are not afraid of cold weather”. (Courtesy BBC)