Kampl made his senior debut for Slovenia on 12 October 2012. He scored his first goal on 6 September 2013 against Albania, which Slovenia won 1–0. Kampl scored his second goal in a UEFA Euro 2016 qualifying match against San Marino, the second of their 6–0 win. In October 2016, the Slovenian squad released a statement "condemning Kampl's absence from both games." In response, Kampl stated "Firstly, I always play with pride and happiness for Slovenia and I intend to do so in the future" and secondly, I have made it clear I will be available next time, I just need some rest now."