South Korea's new President Moon Jae-in backed proposals for a collaboration after it was suggested by sports minister Do Jong-hwan.
But North Korean International Olympic Committee (IOC) member Chang Un has dismissed the idea - saying there was not time to negotiate a deal.
The Games in Pyeongchang, South Korea, will take place from 9-25 February.
The two sides have played in the same team before - at the 1991 World Table Tennis Championships.
However, Mr Chang told local media: "It took us 22 rounds of talks to set up that joint [table tennis] team... it took us five months.
"That's the reality we face."
South Korea's sports minister had suggested a joint ice hockey team - even going as far as to suggest they might allow the north to host skiing events - to help make the 2018 games a "peace Olympics".
President Moon, who advocates greater dialogue with South Korea's neighbours, then put forward the idea of a wider unified Olympic squad.
But Mr Chang said the games should not be used for political purposes, adding: "As an expert of the Olympics, it is a little late to be talking about co-hosting. It's easy to talk about co-hosting, but it is never easy to solve practical problems for that. It's the same for forming a joint team for ice hockey."
South Korean officials have said they continue to be open to the idea.
The two sides remain technically at war as the fighting at the end of the Korean War in 1953 did not end with a peace treaty. Tensions have risen recently following repeated missiles tests carried out by Pyongyang.
North Korea has rejected an offer from the South to form a unified team for next year's Winter Olympics.