There had been reports earlier that the explosion caused partial damage, but UN satellite analysts say that almost nothing remains of the 2,000-year-old site.
The United Nations Institute for Training and Research (UNITAR) has posted a satellite image on Twitter taken by its Operational Satellite and Application Programmes (UNOSAT) on Tuesday.
Dedicated to the Palmyrene gods, the Temple of Bel was one of the best-preserved parts of the ancient city, in the desert north-east of the capital Damascus.
"Unfortunately, the images we acquired do show that the main building of the temple has been destroyed," UNOSAT manager Einar Bjorgo told the BBC.
IS seized Palmyra in May from government forces and it is the second temple that has been targeted.