Chad Ryan DeSoto entered the plea during a hearing Wednesday local time, said Carlina Charfauros, a spokeswoman for the Guam attorney general's office.
DeSoto is accused of driving onto a sidewalk and striking seven tourists in an upscale shopping area fronting the Outrigger Guam Resort in Tumon Bay. Authorities say he crashed his car into a convenience store, then got out and started stabbing people.
Three Japanese tourists were killed in the Feb. 12 incident, which shocked the tiny U.S. territory. The tropical island depends heavily on tourism—especially from Japan, which is about 1,500 miles away—and media there followed the case closely as tourism officials worked to swiftly reassure visitors that Guam is safe.
Charfauros said Superior Court Magistrate Judge Alberto Tolentino ordered a psychiatric evaluation to be completed within 10 days. A follow-up hearing was set for March 20.
An attorney for DeSoto in the Guam public defender's office could not immediately be reached for comment.
The three tourists killed were 81-year-old Kazuko Uehara and 29-year-old Rie Sugiyama, who were stabbed to death. Hitoshi Yokota, 51, was hit by a car and died in a hospital two days later. Another 11 people were injured.
DeSoto is charged with three counts of aggravated murder and 11 counts of attempted aggravated murder. Most of the attempted murder charges were upgraded from aggravated assault charges, and one charge was dropped Wednesday because it was a repeat count in the indictment, Charfauros said.
Spokesman Tony Muna of the Guam Visitors Bureau said that more than $100,000 had been raised in a memorial fund to cover costs for the families of victims.
Nearly three-quarters of the 1.1 million visitors to Guam in fiscal 2011 were Japanese, according to data from the Guam Visitors Bureau. South Koreans made up 13 percent, followed by 4 percent from Taiwan.
Oskar Garcia can be reached on Twitter at http://twitter.com/oskargarcia