Alexei Navalny, 43, was reportedly caged for 30 days after organising “unauthorised” protests.
The activist has never suffered from allergies before, his spokeswoman said.
His symptoms allegedly included redness and severe swelling of the face.
Mr Navalny’s spokeswoman Kira Yarmysh posted on Twitter : “This morning at 9.30am, Alexei Navalny was hospitalised with a diagnosis of ‘acute allergic reaction’: severe swelling of the face and redness of the skin.
“The source of the allergic reaction is not determined.
“Throughout his life, Alexei had never experienced an allergic reaction.”
Navalny was jailed last week after organising protests in response to opposition candidates being barred from upcoming local elections.
More than 1,000 people were taken into custody during the demonstrations.
The European Union criticised the “disproportionate” use of force against protesters.
It added the actions of police had undermined “fundamental freedoms of expression, association and assembly”.
A hospital source has told Russian media Navalny’s condition was satisfactory.
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