Mr Johnson described the use of the chemical weapon - also used in the 2018 attack in the Wiltshire town that left one woman dead - as “outrageous,” adding: “We have seen first-hand the deadly consequences of Novichok in the UK.
“The Russian government must now explain what happened to Mr Navalny – we will work with international partners to ensure justice is done.”
The tweet was shared by Sarah Bailey - the wife of Wiltshire Detective Sergeant Nick Bailey who almost died after he was exposed to the substance while responding to the 2018 attack.
Sharing the tweet on Wednesday evening, she wrote: “Justice would be nice. Actions speak louder than words.”
Her husband also shared the Prime Minister’s tweet, with a short message which read: “I have so much that I want to say about this tweet. But I can’t, and I won’t.”
Dep Sgt Bailey has spoken previously of “still trying to pick up the pieces” of his life after becoming seriously ill from the poisoning.
He lost his family home and possessions due to contamination fears.
Det Sgt Bailey was the first person to enter the home of Sergei Skripal after the Russian former double agent and his daughter Yulia were found poisoned on a park bench in Salisbury.
The officer recovered after a two-week stay in intensive care - during which his wife and two daughters feared he would die.
He made a third attempt to return to work in June this year but has described how he “couldn’t deal with being in a police environment” after efforts to return in September 2018 and again in January 2019.
The Skripals also recovered, but local woman Dawn Sturgess died months after the incident at 44-years-old.
She picked up the discarded perfume bottle used by Russian agents to hold the Novichok in the nearby town of Amesbury.
Mr Navalny, a politician and corruption investigator, fell ill on a flight back to Moscow from Siberia on August 20.
He was transferred to Berlin for treatment, with the German government saying testing at one of their military laboratories showed “proof without doubt of a chemical nerve agent from the Novichok group”.
The 44-year-old remains on a ventilator in intensive care.