He announced the news on Twitter.
The Rock announced he was quietly retired from wrestling roughly two months ago on Kelly and Ryan, but this is less a full-time return and more of a cameo. Expect The Rock to cut a "promo", "lay the smackdown" and leave after "electrifying the crowd."
Rumors were flying that The Rock would make it to Smackdown's Fox debut, considering he was the wrestler that coined the phrase Smackdown was named after. The only issue was The Rock's schedule and that issue appears to have been ironed out. "Smackdown is The Rock's show," was a catchphrase of The Rock for years, so it makes perfect sense for The Rock to make his return on such a big, important night for the brand.
The Rock hasn't appeared on WWE television since WrestleMania 32 in April 2016.