The decision, handed down after Dashkevich allegedly breached prison rules, came into force immediately and he was expected to be transferred to the new jail by late on October 30.
Dashkevich, the leader of the youth opposition movement Young Front, was given a two-year prison sentence in March 2011 on charges of hooliganism after an alleged brawl on the eve of President Alyaksandr Lukashenka's disputed reelection.
In August, a court found him guilty of repeatedly disobeying prison rules and sentenced him to an additional year in jail.
His fiancee, Nasta Palazhanka, said the decision to transfer him to a maximum-security prison was aimed at crushing his resolve.
"The goal is to isolate him to the maximum [degree], to make his detention conditions harsher, so that he feels less comfort and faces more restrictions," Palazhanka told RFE/RL. "Even now, he cannot receive visits or parcels because he allegedly violates prison rules, and the situation there will be even tougher."
Dashkevich's trial took place on the Day of Remembrance of the Victims of Political Repressions, which was being marked throughout much of the former Soviet Union on October 30.
Rights groups in Belarus and beyond have denounced a rollback on freedom since authoritarian Lukashenka was reelected in December in a poll foreign observers have denounced as flawed.
Police violently dispersed the mass protests that followed the election, detaining almost 700 people. Dozens of protesters were sentenced to jail terms.
Dashkevich's fate has raised particular concern.
Human Rights Watch said he was being subjected to routine prison abuse, including verbal abuse, arbitrary punishments, and threats of torture, rape, and murder.
The Belarusian rights group Platform wrote to the United Nations' special rapporteur on torture to complain about his treatment in prison.
'Prisoner Of Conscience'
The 31-year-old activist himself is recovering from a hunger strike launched to protest what he said was "inhumane treatment" at the prison penal colony in the southwestern city of Mazyr, where he was transferred in September.
Dashkevich's lawyer said the activist wrote a letter to the director of the colony stating that he refused to comply with certain requirements of the prison administration that he considered illegal.
Amnesty International has declared Dashkevich a prisoner of conscience.
Palazhanka said he is generally feeling well and in a combative mood.
In September, he turned down an offer under which he could apply for a presidential pardon by admitting his guilt.
The same month, he and Palazhanka got engaged.
Apart from a brief conversation during his mother's funeral in April 2011, the couple has not seen each other for two years.
But Palazhanka said they are not losing hope of being reunited in the near future.
"Our plans haven't changed -- we still want to start a family and continue working in this field," Palazhanka said. "Of course it's difficult to predict the future in this country and in this situation, but we are both optimists. Hope dies last, and in our situation is doesn't dies at all."
Dashkevich's current term ends in August 2013.