The league conducted an investigation after microphones picked up a phrase during a game at Toronto against the Tampa Bay Lightning. Online videos appeared to implicate Rielly, but the NHL said he didn't utter a slur.
"League officials interviewed several of the participants in the game -- including Rielly and Meier -- and reviewed audio of the alleged incident," NHL senior executive vice president of hockey operations Colin Campbell said in a statement. "All of those interviewed adamantly denied that Rielly uttered a slur and the audio supported their statements.
"The National Hockey League does not tolerate language or gestures that disparage anyone based upon their race, creed or sexual orientation and continues to work to ensure that our games are played in a welcoming atmosphere for all of our players, coaches, officials and fans."
The league didn't say whether the slur had been used, just that it wasn't uttered by Rielly. The 25-year-old joined Maple Leafs general manager Kyle Dubas at a news conference Tuesday.
"I didn't hear it," Rielly said of the slur. "I know I didn't use that word, and I didn't hear it during play. I did listen to the video.
"I'm not sure if it came from the ice or not. Either way, that word has no place in this building. This is a team that wants to be involved in the community and with the movement."
In the second period of their 6-2 loss to the Lightning, Rielly chased Lightning forward Yanni Gourde into the Toronto zone during a Tampa Bay penalty kill. Meier skated back with them. On the audio carried by Sportsnet in Canada, there is a clear anti-gay slur shouted by someone near ice level.
The Leafs issued a statement from Dubas after the investigation was announced: "The club is aware of the reports surrounding a homophobic slur used during the Maple Leafs versus Lightning game on Monday night. The issue of homophobia is one the Toronto Maple Leaf Hockey Club strongly condemns and takes very seriously. We are in communication with the NHL and are cooperating fully with their office."
Dubas said he was about to leave the rink Monday night when notified of the alleged comments.
"When it came out that it was Morgan who was alleged to have used a homophobic slur, it was surprising to me, to say the least," Dubas said. "I've known Morgan now for five years, and this is a cause that he's supported socially throughout his time here."
The Maple Leafs have been one of the most vocal supporters of the You Can Play organization, which partnered with the team on Feb. 25 for a night that spotlighted the LGBTQ hockey community. A public service announcement featuring Maple Leafs players was shown in-arena during the game.
"The Toronto Maple Leafs are very much a part of You Can Play history," said You Can Play executive director Ryan Pettengill.
A few weeks ago, Rielly asked to be formally involved in the gay pride parade in Toronto this June, Dubas said.
The league has punished players for using anti-gay slurs during games. Andrew Shaw, then with the Chicago Blackhawks, was suspended for one playoff game in 2017 after using an anti-gay slur while seated in the penalty box. Later that postseason, the NHL came under fire for handing Anaheim Ducks center Ryan Getzlaf only a $10,000 fine for what many felt was an anti-gay slur he used in the Western Conference finals.
Information from ESPN's Greg Wyshynski and The Associated Press was used in this report.