The 1,800-year-old monumental arch, which framed the approach to the Roman city, was blown up Sunday, Syria's Directorate General for Antiquities and Museums said, citing eye witnesses in the local community.
ISIS jihadists seized control of Palmyra, a UNESCO World Heritage Site, and the surrounding area from Syrian government forces in May. Since then, the Islamic extremists have beheaded the antiquities expert who looked after the ruins and set about demolishing their architectural riches.
In August, they leveled two temples of immense cultural significance -- the Temple of Bel and the Temple of Baalshamin -- prompting outrage and condemnation from around the world.
The Arch of Triumph, consisting of one large arch flanked by two smaller ones, opened onto Palmyra's elegant Colonnade. The top of the arch was decorated with "beautiful geometrical and plant ornaments," the Syrian antiquities directorate said.
UNESCO calls destruction 'war crime'
UNESCO, the United Nations cultural organization, describes the archway's adornments as "an outstanding example of Palmyrene art."
The organization's director general has called ISIS' destruction of Palmyra's architectural gems "a war crime."
As well as the temples, the terrorist group has also demolished Roman-era funerary towers that were "among the most representative and evocative monuments" at the site, UNESCO said last month.
The militants, however, are unapologetic over their step-by-step trashing of one of the world's most important ancient sites, publishing photos of some of the destruction.
The Syrian antiquities directorate said it was increasingly worried about Palmyra's fate under the extremists, noting that the ancient city "stands for of tolerance and multicultural richness, the things ISIS hates."
The militants have also smashed up other antiquities in the parts of Syria and Iraq that they control.