Named The Playing Field, the temporary structure was commissioned by the Nuffield Theatre to host stage performances during a 17-day arts festival that took place in the city earlier this month.
Assemble – who has previously created pop-up theatres in a Sussex field and a former London petrol station – conceived the 450-seat venue as a hi-tech Tudor theatre with the drama and spirit of a sports arena.
"A central tenet of the project was to widen participation and attract new audiences to the theatre," explained James Binning, one of the architects that makes up the Assemble collective.
"The ambition was to create a dramatic new typology of theatre space – drawing on the architecture, crowd dynamic and match day ritual of football culture. Utilising the aesthetic and architectural language of Britain's football stadia, the auditorium creates a spectacle that occupies an area between theatre and football, " he said.
The team added large wooden gates on opposite sides of the building, allowing members of the public to wander through the space outside of performance times. There was also no raised stage, so performances all took place at ground level.
"The Playing Field is intended as a setting for ambitious and rambunctious ensemble performances," said Assemble's Paloma Strelitz.
"In the day the large stage doors can open and the raked auditorium can form the backdrop to the daily life of the square. The idea is to create a theatrical experience both for those watching productions and for passers-by," she said.
The architects worked with engineering firm Structure Workshop to develop a framework that could be built simply and efficiently, with only a single crane used to lift structural elements into place.
The location in the city square prevented the addition of ground fixings, so a rigid structure was designed to weight the structure down. This involved creating a system of timber modules, supported by criss-crossing steel braces.
"The structural strategy was to create a timber beam-and-post module that could be repeated around the perimeter of the theatre," explained engineer Pete Laidler.
"This module was adapted to turn the corners and span the goal openings without visibly altering the architectural rhythm."
The timber modules create a colonnade around the perimeter of the structure. Steel bracings are picked out in red, creating an affinity with plastic seating and entranceways in the same colour.
The first floor accommodates a technical gallery, while stage lighting is attached to a truss structure suspended overhead.
One of the plays staged in the theatre during the festival was The Saints, a dramatisation of the highs and lows of being a Southampton Football Club fan.
Photography is by Jim Stephenson, apart from where otherwise indicated.