Mr Reynders was one of the first representatives of western governments to visit the Islamic Republic at the beginning of 2014 following the election of the moderate Hassan Rohani as the new Iranian president and the rapprochement between Tehran and the West as a result of a breakthrough in the nuclear dossier.
The nuclear deal struck in July of this year between Iran and six international powers opens the way for the lifting of economic sanctions.
Iran is an ally in the fight against the group calling itself IS, but Tehran also supports the Assad regime in Syria.
Human rights too will be discussed. International human rights organisations complain that executions are still regularly staged in Iran.
Business as usual soon?
With 80 million consumers Iran forms an interesting market for a country like Belgium that lives off its exports. The prospect of the ending of economic sanctions means business ties can be resumed. The Belgian Foreign Trade Minister Pieter De Crem has joined Mr Reynders on the trip.
On Monday Mr Reynders meets President Rohani, Foreign Minister Mohammad Javad Zarif and Economy and Finance Minister Ali Taieb Nia. On Tuesday the Belgians are in Isfahan for a meeting with the local governor and the authorities there.