Sir James Pennethorne (4 June 1801 – 1 September 1871) was a British architect and planner, particularly associated with buildings and parks in central London.

Pennethorne was born in Worcester, and travelled to London in 1820 to study architecture, first under Augustus Charles Pugin and then under John Nash.

At Rome he studied antiquities, and made a design for the restoration of the Forum, which he subsequently exhibited and was elected a member of the Accademia di San Luca.

In 1848 he modified Nash's Quadrant in Regent Street, removing the colonnade and inserting a balcony and mezzanine story.

[2] In 1853 he drew up several different ambitious plans for the laying out of the estate owned by the Commissioners of the 1851 Exhibition at South Kensington, one of which included a relocated National Gallery.

In the event the area was developed piecemeal, and Pennethorne's contribution to the scheme, in his role as architect to the Office of Works, was a simple "junction" building containing offices and lecture theatre, linking the new iron museum buildings with existing structures, which had been adapted for use as an art school.

[6] He was one of the architects invited, in 1862, to submit designs for a memorial to Albert, Prince Consort, but his suggestion for a monument in the form of a Classical mausoleum was rejected in favour of George Gilbert Scott's Gothic construction.