The 19-year-old heartthrob was thrilled when Camilla Foss, 18, invited him and the group on board the vehicle - which she and over 30 her friends had been travelling on while celebrating the end of their exams - during their stop in Oslo and the pair reportedly hit it off as they travelled through the night partying.
The bus party is a pre-graduation tradition in Norway known as the 'Russ' and is a three-week long event.
A source told The Sun newspaper: "Harry was overjoyed to get the invite to jump on the bus with screaming fans. There were loads of girls, but he focused on Camilla. It got pretty sweaty in there."
Camilla first met the British boy band - which also includes Liam Payne, Louis Tomlinson, Zayn Malik and Niall Horan - while dining at the hotel where they were staying in the city and she's been spotted back since, sneaking a kiss from the curly-haired heartthrob.
The source added: "Fans are saying they've been seen kissing and she's been spotted at his hotel."
After the alcohol-fuelled night, Liam revealed his bandmate ended up in a humorous state in his hotel room.
The 19-year-old singer mused: "You should have seen him. At 12.15 the next day he was just lying there, knocked out, naked on the floor of his hotel."
Instead of feeling embarrassed, Harry - who has been pictured with beautiful Camilla on Twitter - couldn't contain his excitement about the night and insisted he was well-behaved, despite him lacking clothes the next day.
He said: "It was fantastic. I was in that bus for three hours. It was the best time ever. As soon as I got on there were three beers poured over my head and people screaming, 'Welcome to Norway!' I did behave."