Mattis said his visit to the site was aimed at making amends for some parts of the war.
"We had promised to help ... so this is America keeping her promise to remediate some of the past," Mattis, whose older brother served in Vietnam during the war, told reporters this week.
"I just want to get eyes on [the site] so when I go back and I talk to Congress, I can tell them my impression with actually having seen the site," Mattis said ahead of the trip.