Fonthill Splendens was a country mansion in Wiltshire, built by Alderman William Beckford; building began in 1755 and was largely complete by 1770. The construction followed the destruction by fire of the previous Fonthill House. The new mansion had a life of only fifty years, being demolished in 1807.

Beckford is usually referred to as Alderman Beckford to distinguish him from his better known son, William (Thomas) Beckford of Fonthill Abbey. Fonthill Splendens was a major country house of its time, but knowledge of it has largely been eclipsed by the story of his son's Gothic edifice, Fonthill Abbey.

As with the earlier building, the mansion was in its time just known as Fonthill House. The name Fonthill Splendens may have been used colloquially but only appeared in print in 1829, some twenty years after the house was demolished.

This substantial mansion was built in the Neo-Palladian style, largely based on the design of Houghton Hall in Norfolk. It seems that a formal architect may not have been engaged. Credit is usually given to a "Mr Hoare, a London builder". Given the closeness of Fonthill to Stourhead, James or George Hoare may have had some connection to the Hoare family.

The building consisted of a nine-bay central block on three floors. Quadrant colonnades connected this to free-standing five-bay pavilions of two floors. Two flights of steps on the north front led to a portico with pediment on four giant Ionic columns. The principal rooms were all on the first floor or piano nobile, set around a vast central hall, the Organ Hall. The basement or 'Rustick Story' floor seems to have been used for everyday living. All services such as the kitchen and brewhouse were confined to the separate wings, internal passageways in the quadrants connecting them to the central block.

Architecturally, the house was considered a rather old-fashioned design, some twenty years out of date compared to the latest styles. The interior, however, was fitted out by the Alderman in the current taste of the 1760s and was extremely richly decorated throughout, with painted ceilings, elaborate chimneypieces, velvet and damask wall hangings, a library, many oil paintings and tapestries.

He also carried out extensive landscaping works to the grounds, where the stream flowing north/south through the grounds was dammed to form Fonthill Lake. A bridge crossed the lake, and on the shore a boathouse and several grottoes were built by Joseph and his son Josiah Lane. The fine North Lodge entrance remains: an archway with pediment over and lodges either side, the facades decorated with vermiculated block rustication. The house seems to have suffered from its low-lying position close to the lake, there being many complaints over the years of it being damp.

Alderman Beckford died in 1770, and thereafter the house was the property of his son William Beckford, then aged ten. He lived in the house for some thirty years before moving to his Fonthill Abbey in 1801.