"Wait... is this my real hair colour?" Witherspoon captioned the post.
It's difficult to remember a time before the Hollywood heavyweight sported anything other than her now-famous blonde locks.
Witherspoon's most referenced beauty phase is without question her stint as Elle Woods in Legally Blonde, as she sported Malibu Barbie waves in the 2001 silver screen hit.
Fast-forward to Big Little Lies and the mother-of-three took on the role of Madeline Mackenzie - another excuse to keep her covetable 'do.
That's why behind-the-scenes snaps of the actress in The Morning Show soon went viral with fans surprised by her transition into chestnut brown territory.
The actress has only adopted the hue twice previously for the big screen in Walk The Line (2005) and The Good Lie (2014).
With news that Witherspoon is set to reprise her most famous role as Elle Woods in a third Legally Blonde movie, it seems she'll be sticking to her newfound style a little while longer...