Johnson (30-3-1), known to MMA fans as "Mighty Mouse," reigned in the UFC for six years and made more title defenses (11) than anyone in the promotion's history. He is No. 9 in the ESPN pound-for-pound men's rankings and No. 2 in the flyweight Top 10.
The 33-year-old from Parkland, Washington, moved to One Championship last year as part of a trade that brought Ben Askren to the UFC from the Asia-based promotion.
Moraes (18-3), a 30-year-old native of Brasilia, Brazil, is in his third run with One's 125-pound belt. His most recent title-winning performance came in January 2019, when he reclaimed the title from Geje Eustaquio. He has not fought since.
Johnson earned his title shot by winning the 2019 One Flyweight World Grand Prix, defeating Danny Kingad in the final in October.
The date and site for this championship fight were announced Sunday night on social media by Chatri Sityodtong, CEO of One Championship.