The Good Morning Britain host tweeted, "I misjudged you, @ArianaGrande & I apologize. You're an admirable young woman & this is a magnificent night."
He initially criticized her in response to a tweet that Katy Perry had sent out, arguing that it would have been nice for Ariana, 23, to have visited the injured victims just like The Queen had.
Emotions run high in periods like this. Last night's concert showed me I had misjudged Ms Grande. https://t.co/fPDfgGLK6r— Piers Morgan (@piersmorgan) June 5, 2017
Ariana flew straight back to the USA after the terror attack, and aside from one tweet where she described herself as "broken," she had remained quiet on social media.
Piers, 52, said that it had struck him as "odd" that she had returned home without visiting her injured fans. He said: "I can 100% guarantee you and I would stay & visit those who had been killed or wounded watching me perform."
Fans rushed to defend Ariana against Piers' comments, but he stood his ground.
Ariana arrived in the UK a few days ago and took time out to visit her injured fans, offering plenty of hugs and taking selfies with them.
Piers had a change of heart after seeing the "One Last Time" singer host the One Love Manchester concert last night at the Emirates Stadium. Ariana took to the stage to perform some of her biggest hits, as well as being joined on stage by some of the biggest music superstars including Justin Bieber, Miley Cyrus, and Katy Perry.
In an emotional speech at the charity concert, an emotional Ariana reached out to her fans and said "I want to thank you all from the bottom of my heart for being here. Thank you for coming together and being so loving and strong. The love that we have here is the medicine that we need right now".
Our thoughts are with all of those affected by the Manchester terror attack.
To donate to the victims of the attack, please click here.
This article originally appeared on Heat World.