The park did not say exactly how the man died.
Park spokeswoman Aly Baltrus said the two were a couple and had gone canyoneering before but never in the Subway. Baltrus said the couple was from out of state.
"They had not been in the Subway before and had some difficulty with route-finding," Baltrus said.
She said the couple had also taken a class on traversing canyons before beginning a planned trip through the park. They had been in the park for a few days before the accident, Baltrus said.
The Subway is Zion National Park's most popular canyoneering route and requires several rope rappels. The park reported that this was the first death in the Subway in recent history, but there are some rescues from there every year. Rangers have had to make four rescues from the Subway so far this year. In 2011 there were eight rescues from there and four in 2010.
Despite the Subway's difficulty, it remains the park's most popular trail, Baltrus said. Those who decide to take it on should be aware of the risks, Baltrus said.
In a press release, Zion Superintendent Jock Whitworth expressed sympathy for the man's family.
"The Subway is deceiving," Whitworth said. "It is a very popular trail, but very difficult the 9-mile hike requires rappelling and ascending skills, extensive route finding experience, and swimming through several cold and deep pools. Unfortunately, its location inside the Wilderness also means that rescues are not always possible or timely enough. Sound decision making and problem solving are critical."