Her comments come after an Instagram photo Adele shared last week to mark her 32nd birthday sent the internet into a spiral.
The “Someone Like You” hitmaker was noticeably thinner and her dramatic transformation sparked a debate about body image.
Some commentators claimed that celebrating Adele’s new look elevates the status of slim bodies within society at the expense of those who do not conform to being thin.
But Osbourne threw her support behind Adele, saying: “It was her time to lose weight, that’s all, in her journey, in her life.
“She must have felt, you know what, I’m going to try and lose some weight, for whatever reason — health, I am sure.”
Osbourne urged everyone to be happy for the singer because it was a personal decision.
“She looks absolutely fantastic. I’m happy for her and everybody should be happy for her, because it was her choice. She didn’t do it for anybody else but herself. And so, everybody should be happy for her.”
Osbourne went on to reveal that when she was 100 pounds overweight she was more comfortable when she was with “bigger women” because she “felt like we had something in common.”
“We never spoke about it, but there was this underlying connection that we had,” she added.
Osbourne, who is married to Black Sabbath rocker Ozzy Osbourne, had gastric band surgery in 1999 which helped her lose weight.
She added: “When really big women say they’re really happy in their body, I don’t believe them.”
Reflecting on her own battle with body image, the mother of three explained: “I was really, really big and I wasn’t happy.”
Osbourne said she might have seemed happy on the surface, “but at night in bed, alone, I was very unhappy.”
CNN has reached out to Adele’s representatives for comment.
In Adele’s post last week, the Grammy winner paid tribute to health workers, writing in her caption: “I’d like to thank all of our first responders and essential workers who are keeping us safe while risking their lives! You are truly our angels.”
She added: “2020 okay bye thanks x.”