The bout headlines the UFC on Fuel TV 7 card.
Barao (29-1-0, with one no contest) will be making the first defense of the belt he captured on July 21 with a unanimous decision victory over former WEC featherweight champion Urijah Faber.
An interim title became necessary due to a knee injury suffered by lineal champion Dominick Cruz, who has been sidelined for nearly one-and-a-half years.
If McDonald (15-1) leaves the Octagon victorious Saturday, he will supplant light heavyweight titleholder Jon Jones as the youngest champion in UFC history. McDonald is 22 years old; Jones was 23 when he lifted the 205-pound belt from Mauricio Rua in March 2011.
Swanson came in at 146 pounds, while Poirier made the fight official by checking at 145. The winner of this fight is expected to receive strong consideration for a title shot.
Poirier, who took the fight on short notice after Dennis Siver withdrew due to injury, has a professional record of 13-2. Swanson (18-5) has won three fights in a row.