The 600-year-old Namdaemun (South Gate), which is listed as "National Treasure Number One", has been painstakingly rebuilt at a cost of USD 24 million.
The city landmark, also known as Sungryemun, was one of four gates built to protect the city when it was the capital of the Joseon Dynasty, which ruled the Korean peninsula from 1392 until the Japanese occupation in 1910.
"Sungryemun is a symbol of national spirit and identity and the face of the Republic of Korea," President Park Geun-Hye said in a speech at the opening ceremony on Saturday, describing it as a "very happy moment".
Park, wearing a traditional Hanbok robe, and other participants unveiled a large wooden tablet bearing the gate's name.
Also Read
They then swung open its studded wooden door watched by guards wearing traditional clothes and carrying swords, spears, and bows and arrows.
The restoration project -- one of the longest and most expensive ever undertaken in South Korea -- involved more than 1,000 craftsmen who used traditional tools to restore the gate to its former splendour.
Fortress walls that were destroyed during Japan's 1910-45 colonial rule were returned to their original form.
The largely wooden structure, which survived the devastation of the 1950-53 Korean War, was almost reduced to ashes by a disgruntled 69-year-old man with some paint thinner and a cigarette lighter on February 10, 2008.
He torched the gate after claiming he had received insufficient compensation following the expropriation of his land as part of an apartment-building project in Seoul's northwestern satellite city of Koyang.
The arsonist was eventually sentenced to 10 years in prison.