The individual to be expelled was not named but was of the "Second Secretary" rank within the Russian embassy in in Rīga, Rinkēvičs told journalists at a specially-convened press conference.
"We have ruled on the expulsion from Latvia of the Russian embassy's second secretary," the minister said, in addition explaining that the Interior Ministry had also ruled on the adding of a business person heading an unnamed Russian company doing business in Latvia to Latvia's entry blacklist.
According to Russian embassy sources, the business person to be blacklisted is an employee of Russian state airline Aeroflot.
The decision on the diplomat had been taken because the individual was deemed to be in breach of the Vienna Convention on diplomatic relations, the minister explained, without explicitly stating that they were suspected of espionage.
The story came as part of a co-ordinated show of solidarity among NATO and EU members, with other countries announcing similar measures within minutes.
The UK ambassador to Latvia, Keith Shannon, was quick to express his gratitude.
Meanwhile Russia will respond to Latvia's decision about expulsion of a Russian embassy staffer by doing the same, Russian Ambassador to Latvia Yevgeny Lukyanov told the LETA newswire.
In Estonia, Foreign Minister Sven Mikser announced that Estonia too has decided to expel a Russian diplomat, reportedly the military attache.
"As the activities of the employee of the Russian Embassy are not compatible with the diplomatic status agreed in the Vienna Convention, we have decided to expel the diplomat. We informed the Ambassador of the Russian Federation today of the diplomat’s expulsion from Estonia," Foreign Minister Mikser said.
"The Salisbury attack is not only a dispute between the UK and Russia, it threatens international security and undermines international law," Mikser added.
In Vilnius the ambassador of Russia to Lithuania Alexander Udaltsov was summoned to the Ministry of Foreign Affairs where Vice-Minister of Foreign Affairs of Lithuania Darius Skusevičius handed him a protest note about the Salisbury assassination attempt, informing him that three employees of the Russian Embassy had been named undesirable for activities incompatible with their diplomatic status.
The Ministry of Foreign Affairs of Lithuania also asked the Migration Department to ban another 23 persons from entering Lithuania "in the face of threats to public order and internal security."
France, Germany and Canada announced the expulsion of four diplomats each, while the United States is to expel 60 Russian embassy staff.
The move comes just a few hours after the Russian embassy in the United States was suggesting America should use its influence to "Calm Downing Street" while it protested its innocence in connection with the poisoning of Skripal.
Help London. Calm Downing Street.— Russia in USA ?? (@RusEmbUSA) March 26, 2018
Strategic stability of the entire world depends on Russia????US relations. They should not be taken hostage by clearly staged stories
➡️ https://t.co/YNqGgM89xY pic.twitter.com/ju849lRoym
That plea appears to have been ineffective.
Story will be updated.