As a key backer of Syrian president Bashar al-Assad, Iran has been excluded from earlier talks.
But that is changing.
Iran's foreign minister will join his counterparts from Russia, the United States and Saudi Arabia for negotiations aimed at ending the nearly five-year war in Syria.
Egypt, Iraq, Qatar, Lebanon, France and European Union representatives will also attend the talks in the Austrian capital, Vienna.
Iran's inclusion marks a crucial shift after it was excluded from earlier talks mainly because of opposition from the United States and Iranian rival Saudi Arabia.
The Saudis, along with Qatar and Turkey, say Syrian president Bashar al-Assad must leave power as a precondition for peace.
But Iran, which has been backing Mr Assad along with Russia, says any solution must involve him.
Iranian deputy foreign minister Hossein Amir-Abdollahian says there is no other choice.
"We believe that the solution in Syria can only be a political one. Americans and foreign players in Syria have no choice but to accept the realities there. Mr Bashar al-Assad and the Syrian government have the necessary readiness for talks with those insurgents who are committed to a political path."
The Syrian National Coalition, an opposition group based in Turkey and backed by Western powers, says Iran's participation in the talks will undermine the political process.
But German foreign minister Frank-Walter Steinmeier says, while not a breakthrough, getting the key parties to meet is a necessary first step.
"This seems like not much, but, in light of the problems we had in recent weeks and months, getting the most important participants down at one table is a lot."
US Deputy Secretary of State Antony Blinken has cautiously welcomed Iran's participation.
"It's hard to imagine a solution to the Syrian crisis without their participation. We hope that that participation can be positive and that they will join in the effort to create a political transition that leads to the end of the conflict, but that's an open question. But the only way to test the proposition is to actually engage with them and to see what they are willing to do, in terms of the influence that they have with the Assad regime."
It is not clear whether the Syrian government or the opposition have been invited.
Neither side was present at the last talks in Vienna.
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