The fighters first revealed the information Tuesday on their respective Twitter accounts, after receiving permission to do so from UFC.
Nelson wrote: "Ok guys you can buys tickets to @UFC 143 now since I am going to be on the card fighting Werdum. I can now officially talk about it!"
Nelson halted his two-fight skid Oct. 29 with a third-round TKO of Mirko Filipovic at UFC 137. The former IFL heavyweight champion is 16-6 (3-2 in UFC).
Werdum, who recently has competed in Strikeforce, returns to UFC for the first time since October 2008, when he was knocked out in the first round by Junior dos Santos.
At the time of that fight, Werdum was riding a two-fight winning streak, while dos Santos was making his UFC debut. Dos Santos has yet to lose a fight in UFC and currently holds the promotion's heavyweight title.
Werdum's return to the Octagon became official Dec. 15 when UFC president Dana White announced that Strikeforce was abandoning the heavyweight division. UFC and Strikeforce are both owned by Zuffa LLC.
After receiving permission to discuss his upcoming fight, Werdum (14-5-1) took to Twitter Tuesday.
Werdum, who compiled a 3-1 mark in Strikeforce, announced his return to UFC via a streaming video link from his gym in Venice, Calif.
It will not be known whether Nelson-Werdum will appear on the main card at UFC 143 until the promotion makes an official announcement.
Current welterweight titleholder Georges St. Pierre was scheduled to defend against Diaz, but was forced to withdraw due to injury.
Franklin McNeil covers mixed martial arts and boxing for ESPN.com. Follow him on Twitter at www.twitter.com/Franklin_McNeil.