The five-piece, comprising of Max George, Nathan Sykes, Tom Parker, Jay McGuinness and Siva Kaneswaran, also hit No.1 in the Canadian iTunes chart this week.
Jay McGuiness said: 'We're proud that Glad You Came is doing so well. The British are smashing it at the minute in the US and it's great to feel like we're helping steer that.'
The band, who have just finished their US tour, will see their profile rise still further when they perform live on The Jay Leno Show later this month.
Speaking to MTV about their success, Max George said: 'Obviously, it feels great. We've been over there and gave it a go. We did a few TV shows and had a sell-out small tour but we never expected to be top five as quick as we have.'
'We have just got to keep at it now. We've not broken America yet and we've just got to keep pushing it but it feels great.'
George claimed that he was astonished when their Stateside tour sold out, saying: 'We didn't expect that. We would turn up at every gig that we did and it was absolutely rammed and they were singing every song - we can't really get our heads round it to be honest.'
Max revealed that UK audiences get rather more worked up when it comes to their shows, explaining: 'With the US crowd, they love music over there and they really get into it when we are performing and literally bounce off you.'
'Here [in the UK], they are just crazy in the sense of they scream in your face and they wanna rip your clothes off which is brilliant.'
Meanwhile, the group have promised to show now Justin Bieber a good time to celebrate his 18th birthday when he is next in the UK.
Speaking to capital FM Tom said: 'He's in America, but when he comes over here we're planning on taking him out and showing him a good time.'
'They'll be some YouTube footage of him tied to a lamppost at 3am,' added Jay.
Meanwhile One Direction are currently at No.50 in the official US Billboard Hot 100 Chart with their single What Makes You Beautiful.