Crowds greeted Mr Ahmadinejad on his arrival in Beirut, mostly at the urging of the Iranian-backed Hezbollah group.
He is to address a rally in a Hezbollah area where Iran funded reconstruction work after Israeli bombing in 2006.
On Thursday, in a move likely to anger critics, he will tour villages on Lebanon's tense southern border, where Hezbollah fought a devastating war with Israel in 2006 which left 1,200 Lebanese and 160 Israelis dead.
Mr Ahmadinejad's motorcade was showered with rice and flowers on its way from Beirut airport to the presidential palace.
"We support a strong and unified Lebanon. We will always back the Lebanese government and its nation," he said, standing beside President Michel Suleiman.
But he said Iran stood ready to help Beirut confront any Israeli aggression.
"We will surely help the Lebanese nation against animosities, mainly staged by the Zionist regime," he said, in reference to Israel.
Many people are alarmed at the visit, as Iran backs the Shia Hezbollah group, an avowed enemy of Israel.
Speaking during a visit to Kosovo, US Secretary of State Hillary Clinton said that Washington rejected any efforts "to destabilise or inflame tensions" in Lebanon.
"We would hope that no visitor would do anything or say anything that would give cause to greater tension or instability in that country," she said.
Even members of Lebanon's pro-Western parliamentary majority have called the visit a provocation, saying Mr Ahmadinejad was seeking to transform Lebanon into "an Iranian base on the Mediterranean".
Israel accuses Iran of supplying Hezbollah with rockets and other ammunition, but officials close to the group stress instead Iran's support for reconstruction.
They say they have spent about $1bn (£0.6bn) of Iranian money since 2006 on aid and rebuilding.
"Ahmadinejad has done a lot for Lebanon, we are here to thank him," 18-year-old engineering student Fatima Mazeh told the Associated Press news agency.
"He's not controlling Lebanon. Everyone has a mind and can think for himself. We are here to stand with him during the hardest times."
But elsewhere in the country, the group and its international backers are viewed with suspicion by some.
"I am disgusted by this visit," Mona, a 23-year-old Christian, told the AFP news agency. "They refer to [Ahmadinejad] as a saviour, but all he has brought us is trouble."
After talks with President Suleiman and Prime Minister Saad Hariri, who leads the Saudi- and Western-backed coalition, the Iranian leader will meet Hezbollah leader Hassan Nasrallah.
On Thursday, he is expected to tour the border towns of Bint Jbeil and Qana, the scene of some of the worst fighting in 2006.
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.