Reigns was in the ring to sign a contract for his match with Finn Balor that was agreed to last week, but Baron Corbin attacked the Demon King.
Cena came to the ring to help Balor, but he also saw an opening to get the match he wanted with Reigns. The Head of the Table had already signed the contract, so the leader of the Cenation took it upon himself to put his signature next to Reigns' for the match.
The match will likely headline the Aug. 21 event, which is set to take place at the Las Vegas Raiders' Allegiant Stadium.
Cena made a surprise return to WWE at Money in the Bank by interrupting a post-match promo by Reigns after The Tribal Chief beat Edge to retain the universal title.
He then appeared the next night on Raw to lay down a challenge to Reigns for SummerSlam.
Money in the Bank marked Cena's first on-screen appearance for WWE since WrestleMania 36 in 2020 when he lost a Firefly Fun House match to "The Fiend" Bray Wyatt.
Cena took his leave from WWE in order to focus on his burgeoning acting career, taking on roles in blockbuster movies and series, such as F9, The Suicide Squad and Peacemaker.
During Cena's absence, Reigns has established himself as the clear face and top star of WWE, thanks primarily to a fantastic heel run that began at last year's SummerSlam.
By this year's SummerSlam, Reigns will have held the Universal Championship for nearly a full year, running through the likes of Wyatt, Braun Strowman, Edge, Daniel Bryan, Jey Uso, Cesaro, Rey Mysterio and others en route to his match against Cena.
The only televised singles match between Reigns and Cena to date came back at No Mercy 2017 when Reigns beat Cena by pinfall.
While the match and rivalry were solid, both Reigns and Cena were babyfaces at the time, which made it difficult for the fans to get fully invested.
Now, Reigns is an obvious heel who is generating heat, while Cena is a babyface who is coming off one of the biggest pops in recent memory at Money in the Bank.
On top of that dichotomy, Cena is in search of his 17th career world title reign, which would break his tie with Ric Flair for the most all-time, adding yet another layer to the match.
Reigns vs. Cena is one of the biggest matches WWE can possibly put on currently, and it is now set in stone to happen at SummerSlam.
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