Former UFC lightweight champion Eddie Alvarez (29-6-1) is now officially a member of ONE Championship's roster.

Alvarez's contract with the UFC expired following his July loss to Dustin Poirier and rumors emerged in September that he was in talks with the Singapore-based promotion.

ONE Championship CEO Chatri Sityodtong recently teased over the weekend that he was going to announce one of the biggest free agent signings before it was confirmed by him Monday night that it was indeed Alvarez, as he posted a video of him with the contract.

"Hey guys, the moment of truth has arrived, I know you heard the rumors and they are true," Alvarez said in the video. "We got an official contract from ONE FC and we're all signed up."

"I want to thank Chatri and everyone at ONE FC, super professional. This couldn't have went any smoother, didn't take too long and everyone's happy. I'm super happy, 'The Underground King' is coming to ONE FC. And in preparation, I left that shelf empty. You know what that's for, I'm coming, get ready," he said.

It will mark the fourth major MMA promotion that Alvarez will have fought under. The Philadelphia native competed in DREAM's inaugural Lightweight Grand Prix but could not take part in the final due to a knee injury.

He would later join Bellator MMA in 2009 where he became a two-time lightweight champion before making the move to the UFC in 2014. Although he suffered a unanimous decision loss to Donald Cerrone in his debut, Alvarez would defeat the likes of Gilbert Melendez and Anthony Pettis before shocking Rafael dos Anjos to win the lightweight title.

The 34-year-old then got a money fight with Conor McGregor as the pair headlined the promotion's first-ever card at Madison Square Garden in November 2016 at UFC 205. However, Alvarez was outclassed and knocked out in the second round, losing his lightweight title in the process.

A no contest with Poirier followed before Alvarez got back to winning ways with a Fight of the Year candidate against Justin Gaethje where he won by way of third round knockout. His rematch with Poirier at UFC Calgary in July ended in a second round knockout defeat.

Alvarez is undoubtedly one of the biggest signings in ONE history and it's hard to see him not challenging for the lightweight title soon, however, he may be given a tune-up fight of sorts first.

Australia's Martin Nguyen was the most recent lightweight champion before vacating it due to injury. As it stands, former champion Eduard Folyang and Singapore's Amir Khan will fight for the vacant belt at ONE: Conquest of Champions which takes place Nov. 23 in Manila, Philippines.

Japan's Shinya Aoki, who recently defeated Ev Ting, is expected to face the winner of the bout in the promotion's first-ever visit to Tokyo on March 31.

Interestingly, Alvarez and Aoki have fought each other twice in the past with both fighters having one win each. Maybe a trilogy fight could happen for the ONE lightweight title.