The studio, which has renovated other Grade II*-listed buildings such as the BAFTA headquarters and Portsmouth's Guildhall, was selected from a shortlist of seven.
Renovation to transform headquarters into an "accessible and sustainable building"
The aim of the refurbishment will be to modernise the listed building.
"The lead architect is tasked with the practical challenge of bringing a listed building up to scratch, from an accessibility and sustainability perspective," said Simon Allford, RIBA president.
"Benedetti Architects will lead a design team tasked with transforming the RIBA's Grade II* listed headquarters into a dynamic, accessible and sustainable building."
The refurbishment will see Benedetti Architects turn the building into a cultural hub with flexible workspaces for all of its employees, with the aim of future-proofing the building.
"We adore 66 Portland Place and agree it needs comprehensive improvements to be fit for purpose to secure its sustainable future culturally and environmentally," said Renato Benedetti, director of Benedetti Architects.
Construction set to start in 2024
A key requirement of the refurbishment, which is set to start in early 2024, is to rethink the entrance of the building. Four gallery spaces in the building – a public gallery, a members' gallery, a collections gallery and a public affairs gallery – will also be restored.
The studio will also be required to restore some of the building's most significant spaces, including the Jarvis Auditorium, Aston Webb and Lutyens Rooms and Florence Hall.
Benedetti Architects saw off competition from architecture studios including David Kohn Architects, Hall McKnight and Roz Barr Architects to redesign the building, which was designed in 1934 by RIBA Royal Gold Medal-winning architect George Grey Wornum.
The refurbishment is just one aspect of a wider restructuring of RIBA's London headquarters. Alongside the 66 Portland Place refurbishment, RIBA is preparing to sell 76 Portland Place, a property that was renovated by architecture studio Theis + Khan in 2015.
Photography is courtesy of RIBA.