Photographed by a fan of our spy photography agency, this prototype uses a lot of false body panels to disguise the new sedan's shape and detailing.
Despite that, we can see that the new S90 will share design elements with the XC90, including headlights that are bisected by a horizontal styling line.
Rumours suggest that the new S90 will debut towards the end of this year before going on sale some time next year, at which point it will replace the S80 sedan that debuted back in 2006.
A closely related V90 wagon will join the range down the track, and will displace the V70 from the Volvo family.
Under the skin, the new S90 will use Volvo's new Scalable Product Architecture (SPA), which made its debut under the XC90.
Given the close relationship between the upcoming S90 and the just released XC90, it's virtually a given that the new sedan will feature the same drivetrain options as the large crossover.
This means that the four-, six- and eight-cylinder engines used in the current car will be replaced by the turbocharged four-cylinder petrol and diesel motors employed in the XC90.
Australia-bound XC90s are currently available with either the D5, T6 and T8 drivetrains. The 165kW/470Nm D5 employs a 2.0-litre twin-turbo diesel engine, while the 235kW/400Nm T6 features a turbocharged and supercharged 2.0-litre petrol motor.
The top-shelf T8 takes that twin-charged 2.0-litre engine and adds in plug-in hybrid components for a total of 303kW of power and 640Nm of torque.
In overseas markets, a 142kW/400Nm D4 diesel and a 197kW/350Nm T5 2.0-litre turbo-petrol are also available.