"We will participate in the Afghanistan meeting. At what level, I don't know yet, but we will participate," foreign ministry spokesman Hassan Ghashghavi told AFP.
His Dutch counterpart Bart Rijs also told AFP that Iran will take part in Tuesday's conference.
The Iranian announcement marks a sharp change from the policy adopted towards the administration of president George W. Bush.
Iran stayed away from the last international conference on Afghanistan in Paris in December when Bush was still in office.
US Secretary of State Hillary Clinton had appealed to Iran earlier this month to join the conference which will also be attended by other NATO governments, and "key regional and strategic" nations, notably Pakistan.
Clinton is one of the main instigators of the conference to discuss security and reconstruction in Afghanistan, where a persistent insurgency by the Taliban militia has become a mounting threat to the NATO-led International Security Assistance Force.
"It's a positive message that Iran is sending … to indicate that it prepared to help the United States to restore calm to Afghanistan," Iranian analyst Mashallah Shamsolvaezine told AFP.
But he cautioned that Shiite Iran would be careful not to further antagonise the hardline Sunni Muslim Taliban, with which it had hostile relations when the militia held power in Kabul between 1996 and 2001.
"Iran knows that ISAF forces are guests at best and that they will eventually leave Afghanistan while the Taliban forces are an enduring part of the Afghan landscape," Shamsolvaezine said.
"Tehran wants to act in a way that does not make enemies of the Taliban."
Since President Barack Obama took office in January, he has made a series of diplomatic overtures towards Iran in a sharp break with the policy of the Bush administration which dubbed it part of an "axis of evil".
In a video message to Iranian leaders marking the Persian New Year, Obama called for a "new beginning" between the two countries which have had no diplomatic relations since the aftermath of the Islamic Revolution of 1979.
The two governments have a shared interest in restoring stability to Afghanistan.
Obama has made the conflict there his top foreign policy priority, above the war in Iraq.
Iran in turn has suffered badly from the effects of surging Afghan opium production, with cheap and readily available heroin fuelling a sharp rise in drug use.
It also has close ethnic and religious ties with its neighbour.
During a visit to Brazil on Thursday, Iranian Foreign Minister Manouchehr Mottaki called for a "regional solution" to the crisis in Afghanistan, Iran's Fars news agency reported.
"The goal of Iran is to have stability and peace in the region because that is the basis for any progress," he said.
Iran will also attend another meeting on Afghanistan to be held in Moscow on Friday, the foreign ministry spokesman said.
Washington has confirmed that it will send a top diplomat to that meeting, which is being held under the auspices of the Shanghai Cooperation Organisation, a grouping of central Asian states that also includes China and Russia.
The Hague conference is officially being co-hosted by Afghanistan, the United Nations and the Dutch government. It will be opened by UN Secretary General Ban Ki-moon.
About 80 countries and 20 international organisations and agencies have been invited.
