Gabe Watson, who spent 18 months in an Australian prison for the death of his wife during a honeymoon diving trip, has been acquitted of her murder by a court in Birmingham, Alabama.
A judge halted the trial even before the defence had presented its case, saying prosecutors lacked evidence to prove Mr Watson, 34, intentionally killed his wife to claim a life insurance policy.
He had faced life in prison without parole if convicted.
Gabe and Tina Watson were diving on the Great Barrier Reef in 2003 when she drowned. Mr Watson pleaded guilty in Australia to manslaughter involving negligence.
The circuit judge Tommy Nail agreed with defence arguments that prosecutors failed to show Mr Watson intenionally drowned her for her insurance money. The only eyewitness testified that he believed Mr Watson was trying to save his wife.
The state's evidence was "sorely lacking" and did not prove Mr Watson had any financial motive. Jurors never got to deliberate.
"I don't think anyone knows for sure what happened in the water down there," said Mr Nail, who repeatedly clashed with prosecutors during both the trial and earlier hearings.