Hariri made the surprise announcement in a pre-recorded message on a Saudi-owned TV station.
Last week, Saudi Minister for Gulf Affairs Thamer al-Sabhan predicted on Lebanon's MTV station that 'astonishing developments' were coming for Lebanon.
Shiite Hezbollah dominates Lebanon, but it has sought not to provoke the Sunni community, which in turn has avoided crossing the guerrilla force.
The fear among some Lebanese now is that Saudi Arabia will upset that balance, trying to compensate for its losses in proxy wars elsewhere.
In Syria, Hezbollah and other Iranian-backed fighters allied with President Bashar Assad's forces have recaptured large areas and are working to secure a much-prized land corridor stretching from Tehran to the Mediterranean through Iraq, Syria and Lebanon.
By contrast, Saudi Arabia has been stuck in a fruitless war in Yemen against Iranian-backed Shiite rebels, and a Saudi bid to isolate Qatar has failed to achieve its goals.
Hariri tweeted that he was 'honored to visit' the king in his office - and some of his supporters tweeted back, telling him to take a selfie raising his left hand as a signal that he's OK.
His visit comes after the shock resignation of Lebanon's Prime Minister Saad Hariri who made the announcement in a broadcast from Riyadh on Saturday.The two camps support rival sides in countries across the region, worsening conflicts in Yemen, Syria and elsewhere.Lebanon has been on the verge of blowing up into full scale violence, and only compromise by Lebanese parties has stopped it doing so in a country still haunted by memories from its own 1975-1990 civil war.Macron, on his debut visit to the Middle East, made the surprise announcement at a news conference in Dubai that he would meet with Crown Prince Mohammed bin Salman.'It was decided to go on this visit to Riyadh to see the Crown Prince, first, it is in order to have a first meeting with him, but also to discuss regional questions, in particular Yemen and Lebanon, he said.'I will also emphasize the importance of Lebanese stability and integrity.'Officials in Lebanon are now reportedly planning to work with foreign states to secure the politician's return to the country in the latest twist in an extraordinary resignation.'Lebanon is heading towards asking foreign and Arab states to put pressure on Saudi to release Prime Minister Saad al-Hariri,' said the official, who declined to be identified because the government had yet to declare the initiative.