The rooms, which will open on Monday on the gallery's principal floor, contain 160 works, two-thirds of them splendidly restored.
They include 20 paintings dusted off from the vaults to be revealed for the first time to the public.
Among the jewels on show are Rembrandt's Artemis, portraying his wife, Saskia, which after restoration seems to emit its own light; a collection of Van Dycks, including a portrait of the Englishman Sir Endymion Porter with the artist; and many Rubens masterpieces, including The Three Graces and The Judgement of Paris.
One room is devoted to Rubens' equestrian portraits, anticipating a style favoured by Velazquez, with whom Rubens worked at the Spanish court.
Another room contains a dozen Rubens cartoons on mythological themes for tapestries commissioned by King Felipe IV for his hunting lodge, and yet another contains 12 Rubens portraits of the Apostles, which show the influence of Caravaggio.
Next week's opening will relieve the worst of what the museum's authorities admit is a miserable chaos of building works, improvised displays and cramped quarters. However, the full improvement and enlargement process still has years to go.
Most of the Prado's Flemish works were commissioned or acquired by Spanish monarchs and aristocrats to adorn their palaces, so the new rooms have been fitted out as palatial salons, rather than as municipal galleries.
The paintings have been arranged decoratively in thematic groups, rather than hung side by side as they were before in a chilly assembly line.
Responding to criticisms that the museum's labelling in the past has been cryptic to the point of incomprehensibility, the new displays have discreet but informative labels.
The latest expansion follows the triumphant inauguration last November of 10 rooms devoted to 18th-century European art.
In December the Spanish culture ministry is to announce expansion plans for the Prado to include an adjoining former cloister and the nearby military museum.