With 99 percent of the vote counted, Santorum had 51 percent to 21 percent for former Massachusetts Gov. Mitt Romney, 14 percent for former U.S. House Speaker Newt Gingrich of Georgia, and 13 percent for U.S. Rep. Ron Paul of Texas, CNN reported.
The state's 40 GOP convention delegates will be awarded proportionately.
Romney and Gingrich did not campaign in Kansas, preferring to concentrate on Tuesday's primaries in Alabama and Mississippi.
Hundreds of Republicans turned out in cool, crisp weather to cast their votes at the caucuses in Wichita, the state's largest city, and Santorum supporters cheered the loudest when Gov. Sam Brownback asked who they backed, the Los Angeles Times said.
"Kansas matters this year in the presidential race," said Brownback.
Republican candidates were vying for 67 delegates in Saturday caucuses -- 40 in Kansas and nine in Guam, the Northern Mariana and another U.S. territory, the Virgin Islands.
Romney has secured 13 of the 26 delegates awarded in Wyoming, CNN reported. Romney won the Wyoming caucuses Feb. 29, but delegates were not allocated until county conventions wrapped up this weekend.
Santorum, who finished second Feb. 29, picked up seven delegates, Paul took four and Gingrich one, CNN said.
Romney captured 84 percent of the vote in the GOP caucus in Guam and 87 percent in the Northern Marianas, winning all nine delegates in each of the U.S. territories in the Pacific Ocean, ABC News reported.
Santorum placed second in Guam, with 7 percent of the vote, and also placed second in the Northern Marianas.
Gingrich won 6 percent of the vote in Guam to 3 percent for Paul. In the Northern Marianas, Santorum placed second followed by Gingrich and Paul.
CNN said its latest count showed 458 convention delegates for Romney, 203 for Santorum, 118 for Gingrich and 66 for Paul. A candidate will need 1,144 delegates to win the nomination at the Republican National Convention in Tampa, Fla., in August.
Alabama, Mississippi, Hawaii and American Samoa will hold their primaries Tuesday.