The couple, who have been dating for almost a year, went public with their romance in March shortly after Witherspoon"s split from Jake Gyllenhaal. The 34-year-old actress has two children - Ava, 10, and 6-year-old Deacon - with her former husband Ryan Phillippe. The pair split in 2008 after seven years of marriage.
According to sources, Toth has 'bonded" with Witherspoon"s children. Whispers of an impending engagement began circulating earlier this month after Toth was spotted making the rounds at several high-profile jewellers in Los Angeles, including Harry Winston.
The news follows a string of celebrity engagements over the holiday season, with Natalie Portman, LeAnn Rimes, Lily Allen, Hugh Hefner, Shania Twain and Ginnifer Goodwin all announcing plans to wed.