MacDonald now meets Douglas Lima in the finals later this year.
The win also resulted in another title defense for MacDonald, the reigning Bellator welterweight champion.
Chael Sonnen, who attempted several times to become UFC champion during his career, announced his retirement following a loss to Lyoto Machida in the co-main event.
The card kicked off with Kyoji Horiguchi earning the Bellator bantamweight title to go along with his Rizin belt with a decision vs. Darrion Caldwell.
Check out complete results from Bellator 222 below:
Rory MacDonald def. Neiman Gracie via unanimous decision (49-46, 48-47, 48-47) in the Bellator Welterweight Grand Prix semifinals
Lyoto Machida def. Chael Sonnen via TKO (strikes) at :22 of Round 2
Dillon Danis def. Max Humphrey via submission (armbar) at 4:28 of Round 1
Patrick Mix def. Ricky Bandejas via submission (rear-naked choke) at 1:06 of Round 1
Juan Archuleta def. Eduardo Danta via KO (strike) at 4:59 of Round 1
Kyoji Horiguchi def. Darrion Caldwell via unanimous decision (48-47, 49-46, 49-46) to become Bellator bantamweight champion
Adam Borics def. Aaron Pico via KO (knee) at 3:55 of Round 2
Taylor Turner def. Heather Hardy via TKO (punches) at 3:53 of Round 1
Valerie Loureda def. Larkyn Dasch via unanimous decision (29-28, 30-27, 30-27)
Robson Gracie Jr. def. Oscar Vera via submission (armbar) at 3:15 of Round 1
Lindsay VanZandt def. Rena Kubota via technical submission (rear-naked choke) at 4:04 of Round 1
Haim Gozali def. Gustavo Wurlitzer via submission (triangle choke) at 4:02 of Round 1
Mike Kimbel def. Sebastian Ruiz via split decision (30-27, 28-29, 30-27)
Marcus Surin def. Nekruz Mirkhojaev via split decision (30-27, 27-29, 29-27)
Phil Hawes def. Michael Wilco via TKO (doctor stoppage) at 5:00 of Round 1
Brandon Polcare def. Brandon Medina via submission (guillotine choke) at 4:33 of Round 1
Kastriot Xhema def. Whitney Jean Francois via TKO (punches) at 3:17 of Round 2
John Beneduce def. Kenny Rivera via unanimous decision (30-26, 30-27, 30-27)