Federal prosecutors had sought the death penalty for Ahmed Muse Salad, Abukar Osman Beyle and Shani Nurani Shiekh Abrar for the February 2011 shooting deaths aboard the 58-foot sloop Quest.
Its owners Scott Adam, 70, and Jean Adam, 66, both retirees from the Los Angeles suburb of Marina del Rey, had set off from New Zealand to fulfill a lifelong dream of sailing around the world.
Their friends Bob Riggle, 67, and Phyllis Macay of Seattle joined them for the ill-fated Indian Ocean leg.
But after two days of deliberation in Norfolk, Virginia -- home to the US Navy's Atlantic fleet -- the jury of seven women and five men today returned a sentence of life imprisonment, court sources said.
The trio had earlier been found guilty of all 26 charges against them, including piracy, which carries a mandatory life sentence, and 22 other counts eligible for the death penalty.
Nearly two dozen people have been convicted in US courts as part of a global crackdown on Somali-linked piracy -- but this was the first case in the United States in which the death penalty was sought.