Mr Fossett has been missing for five months since his single-engine plane disappeared over the Nevada desert.
His wife had filed a court petition to have him declared dead so the process of executing his will could begin.
The judge said the evidence Mr Fossett was dead was "more than sufficient". He described the 63-year-old's fortune as "vast", and an eight-figure sum.
There has been no trace of Mr Fossett since he took off from Yerington, Nevada, on 3 September 2007.
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He was a great spirit and a true adventurer! Ironic, he should get lost on a 3hr. flight over the Nevada dessert. Isn't that where Area 51 is located or very near there? Maybe his Alien friends came und took him home.