Bellingcat, which covers intelligence matters, named him as Alexander Yevgenyevich Mishkin, aged 39, who was charged by Britain last month under the name of Alexander Petrov.
British prosecutors charged Petrov and and another man they named as Ruslan Boshirov with attempted murder for the Novichok nerve agent attack on the Skripals in the English city of Salisbury in March, but said they believed the suspects had used aliases to enter Britain.
Bellingcat last month identified Boshirov as a colonel in the GRU whose real name was Anatoliy Chepiga.
London police said they would not comment on speculation about the real identities of the two men facing charges, in response to a query about the latest Bellingcat report, and repeated they believed the men had used aliases.
Mishkin was born in July 1979 in the village of Loyga in the Archangelsk district of northern Russia, and until September 2014 his registered home address in Moscow was the same as the headquarters of the GRU, Bellingcat said.
"Bellingcat's identification process included multiple open sources, testimony from people familiar with the person, as well as copies of personally identifying documents, including a scanned copy of his passport," the website said.
His GRU rank was unknown, it added.Russia denies any involvement in the poisoning, and the two men have said publicly they were tourists who had flown to London for fun and visited Salisbury to see its cathedral.