BBC News
The two-day visit will include a tour of villages along Lebanon’s tense border with Israel.
The region was devastated during the 2006 war between Hezbollah and Israel and rebuilt partly with Iranian money.
During his visit, Mr Ahmadinejad will meet President Michel Suleiman and Prime Minister Saad Hariri.
He will also attend a rally in the capital Beirut organised by Hezbollah.
It is the Iranian leader’s first visit to Lebanon since he took office in 2005.
The highway that runs from the airport to the centre of the capital Beirut has been decorated with Iranian flags and posters for Mr Ahmadinejad’s visit.
But correspondents say that although the Iranian leader will be welcome in Hezbollah’s strongholds, some members of Lebanon’s pro-Western parliamentary majority see it as a provocation.
Ahead of the visit, Hezbollah’s rivals in government issued a statement saying Mr Ahmadinejad was seeking to transform Lebanon into “an Iranian base on the Mediterranean”.
Last week, US Secretary of State Hillary Clinton voiced her concerns over the visit with Mr Suleiman.
Mr Ahmadinejad’s visit also comes amid tension over a UN inquiry into the 2005 assassination of former Lebanese Prime Minister Rafik Hariri.
The UN tribunal is believed to be close to issuing indictments, including ones naming members of Hezbollah.
Prime Minister Hariri – Rafik Hariri’s son – is under pressure from Hezbollah and Syria to denounce the tribunal.
