The former featherweights will be back in the ring this summer, following his second-round KO victory over the Irishman at UFC 257 in January.
ESPN's Ariel Helwani reported the news of the contract and McGregor, 32, is believed to have agreed his terms weeks ago.
It's looking likely that The UFC plan to host at The T-Mobile Arena in Las Vegas.
UFC boss Dana White recently said when asked if that's where the fight will take place: "I hope so, that's the plan."
And on whether the match could be the headlining event, White added: "I hope so. We all hope so."
Poirier teased that the third match will be a "completely different fight".
The 32-year-old mixed martial artist told UFC commentator Joe Rogan: "It's fighting, man.
"I don't think you ever really have [someone's number].
"He'll make adjustments and it'll be a completely different fight.
"Like the first one and the second was different, the third one is gonna be different as well."
He added: "I'm gonna make adjustments as well.
"I've gotta switch it up, keep things fresh and keep him guessing."