Greece will buy back €31.9 billion of its bonds from private investors at a third of their nominal price, the debt agency said today.
The move will ease its crushing debt load, while it also meets a key condition to receive vital rescue loans.
But the deal will cost slightly more than originally budgeted, and must be approved by bailout creditors who are lending Athens the necessary funds.
The agency said it will pay banks, funds and other private bondholders roughly 33.8% of the bonds' face value.
That is still a highly attractive option for some, as the bonds have been trading well below face value since a major debt writedown in March.
Investors brave enough to have bought Greek debt just a few months ago now stand to make 200% gains.
The buyback will shave some €20 billion off Greece's €340 billion debt, which is now mostly held by its bailout creditors - its European partners and the International Monetary Fund.
The yield on Greek 10-year bonds dropped to about 12.6% today, its lowest since the March writeoff and a sign of greater investor confidence in the country's ability to manage its debt.
The Greek government had no immediate comment on the result of the deal, saying it would await a meeting tomorrow of European finance ministers, who must sign off on the buyback.
Under the terms of the buyback, Greece would spend €11.3 billion of its bailout funds on the bonds - more than the €10 billion initially budgeted. The question remains whether bailout creditors will accept the extra outlay.
Greece is heading for a sixth year of recession. The cumulative drop in economic output is expected to reach 25% - a staggeringly high figure that has been compared with the Great Depression of the 1930s.
The successful buyback deal was a major requirement before Greece could be granted a long-delayed installment of its international bailout funds.
Athens expects Thursday's meeting to approve payment, most of which is earmarked to shore up the country's struggling banks. That is why domestic lenders all decided to participate in the buyback, relinquishing the prospect of long-term gains on the bonds, for the certainty of a big capital injection in the next few days.
Early indications were that creditors would approve the deal and release the funds.
Simon O'Connor, spokesman for EU monetary affairs commissioner Olli Rehn, would not comment directly on the buyback but said: "We expect that the decision on the disbursement will be taken tomorrow as planned."
IMF chief Christine Lagarde expressed satisfaction with the deal. "For the moment I can only welcome the results that have been produced by the debt buyback," she said before the official results were released.
Athens has depended on international rescue loans for the past two and a half years, after it admitted its budget deficit was more than three times the initial forecast and swiftly lost access global bond markets.
In exchange for the funds, the government repeatedly slashed incomes and raised taxes to tame the deficit, creating widespread public resentment that sparked countless strikes and protests - many violent.
