His new release All Over The Place notched up 34,000 chart sales to claim the top spot.
The 16 tracks on the album have been streamed 20.9 million times, the Official Charts Company said.
KSI (Official Charts Company/PA)
KSI told OfficialCharts.com: “Your boy did it! Finally got it, a number one album – mad!
“Thank you to everyone who has supported, I wouldn’t have this without you.
“I’ve been doing music for so long and to finally get this is crazy.
“Official Charts Company. Number one. I never thought I’d ever get one of these.
“I have no right – I’m a YouTuber! Thank you.”
Olivia Rodrigo’s Sour was in second place in the album chart, followed by Faith by Pop Smoke in third.
John Mayer’s Sob Rock was fourth, followed by Planet Her by Doja Cat.
Ed Sheeran topped the singles chart (Yui Mok/PA)
In the singles chart, Ed Sheeran’s Bad Habits came top for the fourth time.
It recorded 101,000 chart sales and 11.4 million streams, more than double the total of The Kid Laroi and Justin Bieber song Stay, which came second.
Heartbreak Anthem by Galantis, David Guetta and Little Mix was third, followed by Jonasu’s Black Magic in fourth and I Wanna Be Your Slave by Maneskin in fifth.
