The 27-year-old had been set for a rematch with Nate Diaz, who beat him in their welterweight fight in March, at the Las Vegas event in July.
After a dispute over promotional duties, UFC president Dana White said organisers were looking for a replacement for the Irishman.
But McGregor said on Sunday: "Happy to announce that I am back on UFC 200."
Writing on social media, he added: "Shout out to Dana White and Lorenzo Fertitta on getting this one done for the fans. Respect."
But US media, including the Los Angeles Times, quoted White as saying: "We haven't talked to Conor or his manager since the press conference. I don't know why he would tweet that."
And McGregor's manager John Kavanagh later added: "I'm 51% optimistic and 49% pessimistic, if you want to know my feelings on it."
McGregor started speculation he was quitting last week when he tweeted: "I have decided to retire young. Thanks for the cheese."
But in a statement two days later, he said: "I am not retired."
White claimed McGregor was withdrawn from the card for refusing to fly to Las Vegas for a news conference and promotional photographs.
In his statement, the fighter claimed he was being asked to do too much promotional work by the UFC and was not able to concentrate on his training.

Everton supporter Michael Cullen, 50, completed the 11-day challenge ahead of his team's FA Cup semi-final against Manchester United on Saturday.
He has now raised more than £53,000 for the Woodlands Hospice in Liverpool, having previously raised £30,000 after swimming the English Channel.
Everton lost the game 2-1, with United scoring a late injury time winner.
Mr Cullen said: "I'm very tired, but elated. On the last day I was very happy all the way and it was great to arrive there.
"I'm back at work tomorrow and I'm actually looking forward to getting back to normality."
Carole Riley, of Woodlands Hospice, said: "We are very proud and appreciative of Michael's efforts.
"Not only has he generated a massive amount of money but he has raised the awareness and profile of Woodlands Hospice and the need for hospice services to such a wide audience.
"We would like to give a massive big and sincere thank you to Speedo Mick and all those who have supported him."

Clubs are required to provide accurate details of training sessions and player whereabouts so they are available for testing at all times.
The Cherries were deemed to have failed to ensure its information was accurate on three occasions.
Manchester City were fined £35,000 and given a warning, after admitting a similar breach earlier this month.
Bournemouth have yet to learn their punishment from the FA, but are expecting similar sanctions.
It is understood part of the charge concerns a player not informing the club of his new home address, while another part relates to members of the Under-21 squad training with the first-team at short notice.

A two-year-old black and white cat named Patch died following a suspected poisoning in Bro Dulas, Llanidloes.
Two other cats in the area were also put to sleep after they became unwell.
RSPCA chief inspector Phil Lewis said: "We don't know whether these incidents were accidental or deliberate or who may have done it."
Patch's owner said: "I am just so angry and devastated. Patch was such a quiet and loving cat."
Mr Lewis added: "We're calling on all cat owners in the Llanidloes area to be vigilant and to take their cat to a vet straight away if they suspect they have been poisoned."
Potential signs of poisoning could include vomiting, difficulty breathing, increased thirst, and appearing to be be sleepy and uncoordinated.

The Serbs lost 6-4 6-4 to Brazilians Marcelo Melo and Bruno Soares in Rio.
The loss came a day after Djokovic was knocked out of the singles by Argentine Juan Martin del Potro.
World number one Djokovic, 29, suffered a surprise third-round defeat at Wimbledon in July when he lost to American Sam Querrey.
The 12-time Grand Slam winner came back to claim his 30th Masters title in Toronto before the Olympics.
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Conor McGregor says he is back on the card for UFC 200, but event organisers claim no new talks have taken place.
[NEXT_CONCEPT]
A football fan who walked to Wembley wearing his swimming trunks has broken his £50,000 fundraising target.
[NEXT_CONCEPT]
Bournemouth have accepted a Football Association charge relating to a breach of anti-doping rules.
[NEXT_CONCEPT]
The RSPCA is urging cat owners to be vigilant after suspected cat poisoning deaths in Powys.
[NEXT_CONCEPT]
Novak Djokovic lost in Rio for the second time as he and 40-year-old team-mate Nenad Zimonjic exited the men's Olympic doubles in round two.