The 22-year old scored the fastest-selling debut album of 2014 so far, shifting 101,000 copies in the past seven days.
The new record, about unrequited feelings for a "guy who didn't love him back", is also the third fastest-selling album over-all, behind Coldplay's Ghost Stories (168,000 copies sold in its first week of release) and Paulo Nutini's Caustic Love (109,000).
"It's amazing!" Smith said of In the Lonely Hour's success: "I was a bit scared a few days ago because I knew that Coldplay's album had come out. I'm so happy!"
Coldplay achieved their sixth consecutive UK number one album with Ghost Stories, which entered at the summit last weekend.
The album is rapidly approaching a quarter of a million sales after adding 75,000 to its total sales this week.
Completing the top five albums are Paloma Faiths' A Perfect Contradiction and Michael Jackson's Xscape.
Just last week, Smith bagged his third number one single with "Stay With Me". His first came last May after he featured on Naughty Boy's "La La La" before his second, "Money On My Mind" followed in February.Reuse content