The two Serie A clubs previously co-owned the 21-year-old, with Icardi spending last season at San Siro.
And, after a campaign that saw him score nine goals in 22 league appearances, Inter have moved quickly to sign the forward outright.
"Mauro Icardi is now 100 per cent an Inter player," the club announced on their official website.
"Internazionale and Sampdoria have reached an agreement to terminate their co-ownership of the striker, who is now fully owned by Inter."
Icardi made his first-team debut for Sampdoria in May 2012 having signed for the club from Barcelona in January 2011.
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