Which alter ego did Beyoncé use prominently during the late 2000s?

Beyoncé used the alter ego Sasha Fierce prominently during the late 2000s.

She introduced the character through the double album I Am... Sasha Fierce, released in November 2008. The project contrasted introspective ballads on its I Am... disc with more energetic, performance-focused songs on its Sasha Fierce disc.

Beyoncé described Sasha Fierce as a stage persona that allowed her to perform with greater confidence and boldness. Songs associated with that era included “Single Ladies (Put a Ring on It),” “If I Were a Boy,” “Halo,” and “Diva.”

The name is sometimes mistaken for Beyoncé’s permanent professional name. It was an artistic persona associated particularly with the 2008 album cycle, not a separate singer. Beyoncé later said she no longer needed to separate her stage identity from her personal self in the same way.