The Foro annonario was built between 1833 and 1840, after the demolition of the ancient church of San Francesco Grande, by municipal engineer Giacomo Santarelli. It was the space where the market, previously held in Piazza della Misura, was held on Mondays and Fridays. The architectural complex presents a square enclosed on two opposite sides by two bodies that make use of the pre-existing wings of the convent; the third side consists of a newly constructed building supported by a portico with seven arches.
On the first floor, some windows are surmounted by lunettes with bas-reliefs depicting personifications of the rivers Ronco and Montone, by sculptor Luigi Righi. The building is closed at the back by an elegant semicircular exedra, supported by Tuscan columns, which once housed the fish market.