Eight monkeys were removed from cages at Symbio Wildlife Park, in Helensburgh, south of Sydney, in May 2010.
They included four cotton tail tamarins and four pygmy marmosets.
Seven of them were found just weeks afterwards and two men, aged 17 and 18, were charged over the alleged theft.
On Saturday a 34-year-old Victorian man returned the final missing tamarin to the owner of the wildlife park.
"The man said a family friend had purchased the monkey over the internet last year," a police statement said.
"Once he learned it had been stolen he made arrangements for its safe return."
The case has now been closed, with no further investigation likely.
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