Witherspoon starred in the 2001 comedy as aspiring lawyer Elle Woods, who sets out to win back her boyfriend and defy stereotypes of blondes.
The Oscar-winner will be joined in the Tuesday reunion by co-stars Luke Wilson, Ali Larter, Selma Blair, Jennifer Coolidge, Holland Taylor, Jessica Cauffiel and Alanna Ubach.
Witherspoon shared a teaser for the event and promised an emotional reunion.
She wrote on Instagram: “Ohmigod you guys!! It’s a Legally Blonde Reunion. We laughed, we cried and we may or may not have done the bend & snap for old times sake!”
Witherspoon reprised the role of Elle in 2003 sequel Legally Blonde 2: Red, White & Blonde, though the film failed to achieve the critical acclaim of the original.
In May, Witherspoon, 44, confirmed a third film was in the works.
The Office star Mindy Kaling will co-write alongside Dan Goor.
The Legally Blonde reunion is set to be made available on the Hello Sunshine YouTube channel at 11.15pm UK time Tuesday.
