The 21-year-old scored three goals in 14 games for Royal Eagles last term.
Ngubo was released by Pirates at the end of 2016/17 without playing a game for the Soweto giants. He was promoted to Bucs' first team from their academy the season before.
"Richards Bay FC have signed Siyanda Ngubo," the club's chairman Sifso Biyela confirmed to KickOff.com.
The second-tier side also recently recruited Mpho Erasmus from Mthatha Bucks and Tshepo Mashigo from Swallows FC.
The club's new coach Brandon Truter has ample attacking options given that ex-SuperSport United forward Thabiso Nkoana is likewise at his disposal.