The MMA fighter was stopped in two rounds in a one-sided fight.
Ngannou, 37, has two losses in two boxing matches, but dismissed the suggestion of returning permanently to MMA.
"You can be sure I'm not done here. It wasn't my day. He was way better than me today" Ngannou said.
"It sucks but it's the game. We all know that."
Briton Joshua, 34, produced a destructive performance against Ngannou, who four months ago took WBC champion Tyson Fury 10 rounds in a narrow defeat.
Ngannou had never previously boxed professionally, having built his fighting career in MMA with the UFC, where he was heavyweight champion.
The Cameroonian was full of praise for Joshua and it is the first time Ngannou has been knocked out in either his MMA or boxing career.
"[Joshua] was quite special because he stopped me," said Ngannou. "He did what Tyson couldn't do.
"In fact, I didn't feel the punch. I think that's what a knockout is about. I don't feel any pain. That's how I know I was knocked out."
What could Ngannou do next?
Francis Ngannou is yet to win in his professional boxing career
In just four months, Ngannou has fought two of the best heavyweight boxers on the planet and been in two of the biggest fights of 2023 and 2024.
However, a few weeks before his fight with Joshua, MMA promotion the Professional Fighters League (PFL) announced he would fight its heavyweight champion Renan Ferreira later this year.
Ngannou left the UFC last year to pursue his boxing ambitions, but signed with its rivals the PFL last May.
Part of his deal with the PFL guaranteed him the freedom to box but Ngannou is now likely to make his long-awaited return to MMA rather than compete in another boxing match.
Ngannou has not fought in MMA since January 2022 in his last UFC fight, when he beat Frenchman Ciryl Gane.