The advert, for a cleaning job near Lillehammer, was posted on the Norwegian government's Labour and Welfare site NAV.
"The client requires a security clearance and the position is therefore available only for ethnic Norwegian persons,” ISS wrote in the advert, which can be seen here.
Carl Fredrik Riise, an advisor at Norway's anti-discrimination ombudsman, said that the advert was in violation of Norwegian law.
"The current job posting is in breach of the discrimination law on ethnicity," he told Nettavisen. "As it stands now, the advert states that people who are not ethnically Norwegian will be excluded from applying for the position. In the ombudsman's view, there is no reason to have such a requirement, as this is not a criterion to obtain security clearance."
ISS's communications advisor Jan Bjørneboe told NRK that the company had long used this exact wording in job advertisements without receiving any complaints.
"We have had the same formulation for 28 years," he said. "But since this has created so much fuss, we must look at the way we express ourselves in the future."