The 29-year-old, who played 13 Tests for New Zealand, will compete for the scrum-half spot with Wales international Rhys Webb from next season.
"I've got plenty of experience to call on, and I always want to use that to help my team-mates," Leonard told the official Ospreys website.
"But I'm of the mindset that you never stop learning, wherever you are in life."
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Leonard's last Test for the All Blacks was the 19-12 win over Wales in Cardiff in 2009 and he left Waikato to join Zebre in August 2013.
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