The anti-corruption campaigner fell ill during a flight and the plane made an emergency landing in Omsk, Kira Yarmysh said, adding that they suspected something had been mixed into his tea.
A hospital source confirmed Mr Navalny was in a serious condition.
Mr Navalny, 44, is a staunch critic of President Vladimir Putin.
In June he described a vote on constitutional reforms as a "coup" and a "violation of the constitution". The reforms allow Mr Putin to serve another two terms in office.
Kira Yarmysh, the press secretary for the Anti-Corruption Foundation, which Mr Navalny founded in 2011, tweeted: "This morning Navalny was returning to Moscow from Tomsk.
"During the flight, he felt ill. The plane made an urgent landing in Omsk. Alexei has toxic poisoning. Right now we are going to hospital."
She added: "We suspect that Alexei was poisoned by something mixed into [his] tea. It was the only thing he drank since morning.
"Doctors are saying that the toxic agent absorbed faster through the hot liquid. Right now Alexei is unconscious."