Bellator MMA officials confirmed the date and venue to ESPN on Tuesday. The televised event will air on Spike TV.
For Sonnen (28-14-1), the bout will mark his first appearance since November 2013 and the first ever under the Bellator banner. His last fight resulted in a first-round TKO loss to Rashad Evans at UFC 167.
The former two-time UFC middleweight title contender was suspended for two years by the Nevada State Athletic Commission in 2014, after testing positive for multiple banned substances.
Sonnen's suspension expired in July. There were expectations he would return to the UFC, but the Oregon native shocked the mixed martial arts community by announcing a multifight deal with Bellator, instead.
Ortiz (18-12-1), the former UFC light heavyweight champion, challenged Liam McGeary for the Bellator title in September 2015, but Ortiz lost via submission in the first round. It marked his last fight, as the 41-year-old Ortiz has dealt with several injury setbacks in recent years and is coming off neck surgery.
During a recent interview with ESPN.com, Ortiz confirmed he only has one fight left on his current deal and admitted this upcoming event with Sonnen might be the final act of his nearly 20-year career.
Sonnen and Ortiz have never fought each other, although they both said they wrestled one another in college.
No other fights have been announced as of yet for the Jan. 21 event.