Seats at a 1924-built theatre in London's West End are being replaced with ergonomic foam, the Ambassador Theatre Group (ATG) said.
Fortune Theatre in Russell Street, Covent Garden, is the first to see new foams in 196 of its 435 seats.
The ATG said deeper seats would provide more leg room and prevent slumped postures, resulting in less fidgeting.
It plans to roll out the new seating to its 39 venues across the UK, including London's Savoy and Lyceum theatres.
The firm said the bases of the seats would remain unchanged.