The actresses starred together in the movie "Election," released in 1999.
"So heart broken to hear this. Working with Jessica on Election was such a pleasure. I’m sending all my love to Jessica‘s family and loved ones," Witherspoon posted on Twitter.
Campbell died "suddenly and unexpectedly" on Dec. 29, leaving behind her 10-year-old son, Oliver, Campbell's cousin Sarah Wessling confirmed on a GoFundMe page. She was 38 years old.
"While coping with this unexpected and tragic turn of events and planning for Oliver’s future, the family is also faced with unexpected cremation, memorial, and probate expenses. All funds collected will go towards meeting expenses and providing for Oliver," Wessling explains.
Campbell's cause of death has yet to be revealed, but Wessling told TMZ her family is awaiting results from the Multnomah County Medical Examiner's Office in Oregon. She passed away in Portland.
Campbell's biggest acting highlight appears to be starring in the Alexander Payne-directed film "Election" in 1999, opposite Witherspoon, Matthew Broderick and Chris Klein. She played Tammy Metzler, the younger sister of Klein's character Paul Metzler.
She also appeared in the series "Freaks and Geeks," "The Safety of Objects" and "Junk," the latter being her last acting role in 2002, according to her IMDB page.
Wessling remembered Campbell as "a true adventurer in every sense of the word."
Fox News' Melissa Roberto contributed to this report.