The 40-year-old was left out of an initial squad of 43 named earlier in June for this year's Olympics after a failed attempt to win selection for the women's eight.
Grainger won gold in the double sculls at London 2012, with now retired partner Anna Watkins, and also has three silvers.
The Scot had switched her focus to the women's eight in May after she and Thornley, 28, failed to make the podium in the double sculls at the European championships.
"While the road to selection for this crew has not been straightforward, I remain confident that they remain a significant contender in Rio," said rowing team leader David Tanner in a statement.
Grainger said it was an amazing feeling to be selected again.
"That feeling doesn’t diminish the second, third, fourth or even fifth time round," she added.
Rowing is one of Britain's strongest Olympic sports, with the country top of the medals table at the 2012 regatta with a total haul of nine including four golds.
(Reporting by Alan Baldwin, editing by Ken Ferris)
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