"The president has signed an amnesty for about 70 political prisoners around the country," presidential advisor Hla Maung Shwe, a senior official at the Myanmar Peace Center, told AFP.
Last week Thein Sein said there would be "no prisoners of conscience in Myanmar" by the end of the year during a speech in London.
The move is the latest gesture of reform by the former general, who has ushered in a period of sweeping change since the end of junta rule in 2011.
Rights groups and officials estimate there were between around 100 and 150 political prisoners in Myanmar ahead of Tuesday's announcement.
Hla Maung Shwe said some of those being released were ethnic minority rebels from northern Kachin state, where the government is working on brokering a crucial ceasefire deal.
He said some prisoners had already been freed.
The military junta, which ruled Myanmar with an iron fist for decades, had long denied the existence of political prisoners.
But hundreds of dissidents have been freed since Thein Sein took power two years ago. Last November he announced a review of all "politically concerned" cases.
Activists welcomed Tuesday's release, but urged the government to go further.
"The president said in Britain that there would be no political prisoners at the end of this year. We would like to say that the promise needs to be implemented precisely," Myint Aung, a member of the Former Political Prisoners group, told AFP.
He said it was not immediately clear if those freed matched the group's list of detained dissidents.
"We will continue calling for the release of the rest," he added.
Thein Sein has been praised by foreign governments for reforms including welcoming Suu Kyi and her political party into parliament.
The quasi-civilian government has also reached tentative peace deals with major armed ethnic minority rebel groups in the country, which has been wracked by civil wars since independence from British colonial rule in 1948.
"Very possibly, over the coming weeks, we will have a nationwide ceasefire and the guns will go silent everywhere in Myanmar for the very first time in over 60 years," Thein Sein said last week.
Hla Maung Shwe told AFP that the the announcement could lead to "more meetings" between the government and Kachin rebels, indicating that the move may have been part of ceasefire negotiations.
In response to Myanmar's changes, the European Union has scrapped most sanctions, except for an arms embargo, and has readmitted Myanmar to its trade preference scheme.
The United States has also lifted most embargoes and foreign companies are now eager to enter the resource-rich nation, with its perceived frontier market of some 60 million potential consumers.
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