Jorgen Watne Frydnes, chair of the Norwegian Nobel Committee, called it "a flagrant violation of human rights and a travesty of justice".
Mohammadi, 52, has been jailed since November 2021 over several past convictions relating to her campaigns against the obligatory hijab for women and capital punishment in Iran.
Her lawyer, Mostafa Nili, said on X on Tuesday: "Mohammadi was sentenced to one year in prison for propaganda against the system."
The Nobel Committee said it "strongly condemns the harsh and unjust sentencing".
Nili said the sentence was in response to calls to boycott parliamentary elections, letters to Swedish and Norwegian lawmakers, and comments made about journalist and student Dina Ghalibaf.
Rights groups said Ghalibaf was taken into custody after accusing security forces on social media of putting her in handcuffs and sexually assaulting her during a previous arrest at a metro station.
Ghalibaf has since been released.
The Iranian judiciary's Mizan Online website said on April 22 that Ghalibaf "had not been raped" and that she was being prosecuted for making a "false statement".
Mohammadi refused to attend a trial hearing in Tehran earlier this month, and in March shared an audio message from prison in which she decried a "full-scale war against women" in the Islamic republic.
She was honoured with the Nobel Peace Prize in October "for her fight against the oppression of women in Iran and her fight to promote human rights and freedom for all".
