Neither candidate campaigned on the island in person.
Results of the all-night count completed Sunday morning Guam time show delegates pledged to Obama with 2,264 votes to 2,257 for Clinton's slate.
The territory sends four pledged delegates and five superdelegates to the National Convention in August in Denver although U.S. citizens on the island have no vote in the November election.
Voters also picked two superdelegates by electing a new party chairman who is uncommitted and a vice chairman who supports Obama.
One other existing superdelegate has favored Clinton and the remaining two are uncommitted.
All-day voting Saturday had people lining up at 21 caucus sites around the U.S. territorial island, which has unexpected importance in a historic Democratic race in which every delegate matters.
At stake on the island 8,000 miles from Washington were four pledged
There was no direct presidential vote, but each candidate had a slate of supporters on the ballot. Guam also has five superdelegates and two of those were being determined in the caucus voting as well - the party chairman and vice chairman.
Slow ballot-by-ballot counting went through the night in the territorial legislative building after votes were hand-carried from the caucus sites.
Presidential caucuses on Guam usually pass without much notice from the candidates.
This time, Obama and Clinton made their case for the territory's four regular delegates with local advertising and long-distance interviews.
Lines formed early at some caucus sites.
Cynthia Estrada of Dededo said she was making up her mind while waiting to vote, but she was leaning toward Clinton.
"She's had the experience," she said. "She's got her husband to help her."
Yona resident Tommy Shimizu said he was voting for Obama delegates.
"It's the fact that he grew up in Hawaii, and I think he can make change," he said. "I think it's time for that."
Clinton and Obama pitched improved health care and economic opportunity as they courted Guam voters from across the international date line.
Both candidates bought local advertising and conducted media interviews. In their protracted race for the nomination, no contest is being ignored.
Both Clinton and Obama say they've got the better health plan for Guamanians.
Obama said in an interview with Pacific Daily News that he would support reexamination of a $5.4 million Medicaid spending limit imposed on the territory. Clinton's husband, former President Bill Clinton, told KUAM radio earlier that his wife would work to remove the cap.
Hillary Clinton also has called for Guamanians to be able to vote in presidential elections.