Rohani is to hold talks with Mohamed ElBaradei, the director of the International Atomic Energy Agency (IAEA), a spokesman said without providing details.
Diplomats said Rohani may be carrying a letter outlining Iran's intention to allow surprise inspections of its nuclear installations.
They said the IAEA may be holding up release of its report, which has already been drafted, in the hope of being able to include such a letter as a sign of Iranian cooperation.
The report is to be released, perhaps Monday, to member states ahead of an IAEA board of governors meeting on November 20, diplomats said.
The United States accuses Iran of secretly working to manufacture highly enriched uranium, which can be used to make atomic bombs, and says Tehran should be judged in non-compliance with the nuclear Non-Proliferation Treaty (NPT).
This judgement by the IAEA would lead to the issue being sent to the UN Security Council, which could then impose sanctions.
Iran's ambassador to the IAEA Ali Akbar Salehi had told AFP Wednesday that Iran would honor its promise to agree to sign an additional protocol to the NPT on surprise inspections. Iran says its nuclear program is strictly peaceful.
He said a letter agreeing to the inspections had already been written and would be handed over to the IAEA before the agency's board meetings.
Salehi said the letter "says Iran is ready to accede to the additional protocol and says please put this intention to the board and we can arrange the signing later."
IAEA officials had said the letter would be coming this week but Salehi said it would not be so soon.
Some diplomats said they were surprised at Rohani's visit, especially since Iran on October 23 filed what it said was a full report on its nuclear program and theoretically has nothing to add.
In any case, a Western diplomat said Rohani "is the principal figure" for Iran in talks with the IAEA on the nuclear issue and "I would think that if somebody is coming and it's him, he would have the authority to speak for the government of Iran."
ElBaradei has already said the IAEA will report that Iran has failed to honor some international nuclear safeguards.
"We reported breaches in the past and there will be new ones in this upcoming report," ElBaradei was quoted as saying by his spokesman Mark Gwozdecky.
Diplomats said however that it will take months to verify the information Iran has supplied and that this could be taken by the board as a reason not to pass judgement on November 20.
"November 20 is an important milestone but we won't be able to finish our work by then," ElBaradei said.
"We will need a few more months, particularly with regard to very complex investigations such as the source" of traces of highly enriched uranium found by IAEA inspectors in Iran, he said.
Iran claims the uranium came from contamination of equipment it had bought abroad and not from producing the material, as the United States charges.
A Western diplomat close to the IAEA said the Iranians may be "cooperating until (the IAEA meeting) November 20 in order to avoid a non-compliance ruling" and when they succeed in this, "then they will break all the rules again."