Hezbollah's al-Manar television aired footage of the meeting, which was also attended by aides to both leaders.
The joint statement also said that Nasrallah and al-Hariri would be in "mutual contact".
The differences between Hezbollah, a Shia Muslim political organisation which runs an armed wing, and member parties of the March 14 parliamentary majority flared into armed conflict in May.
Fighters from Hezbollah and its allies briefly took control of predominantly Muslim west Beirut, prompting an armed response by supporters of the March 14 bloc, including those allied to al-Hariri's Future Movement.
The joint statement released on Monday said that the meeting was "honest and open" and said that the leaders would take "steps to calm the situation in the media and in the street".
The talks between al-Hariri and Nasrallah are being seen in Lebanon as the most significant of a series of meetings between politicians from March 14 and the Hezbollah-led opposition.
Nasrallah and al-Hariri are also committed to implementing the Qatar-mediated deal which in May had called for "national dialogue" talks, Monday's joint statement said.
The next session is scheduled for November 5.
The parliamentary majority is calling for Hezbollah to disarm, in favour of building a stronger Lebanese national army.
However, Hezbollah insists that its weapons are essential to a Lebanese national resistance against Israel, its southern neighbour.
Israel failed to destroy Hezbollah during a 34-day war in 2006, which was sparked by a Hezbollah cross-border raid.
Demands for the disarmament of Hezbollah, which is backed by Syria and Iran, have become increasingly shrill since the 2005 assassination of Rafiq al-Hariri, a former Lebanese prime minister and Saad's father.
Hezbollah is the most powerful faction in Lebanon and its opposition bloc holds veto power over decisions taken in Lebanon's cabinet.
While Hezbollah is not willing to disarm, its leaders have said that the organisation is willing to discuss a defence strategy that would define the role of its fighters.