The 23-year-old joined from Northampton as short-term injury cover in January 2017, and has impressed with three tries in eight matches.
"It great to bring Sion back to Wales. He has worked hard since arriving at Cardiff Blues and has performed well on the field when selected," said head coach Danny Wilson.
"He adds depth to a position that will be very competitive."
Bennett came through the ranks at the Scarlets while Wilson was forwards coach at the west Wales region before moving to England.
"I came here to play rugby and it could not have gone any better.
"Looking ahead I just want a massive pre-season and next year I want that starting spot. That's what everyone wants, I will do whatever the team needs but that's what I am striving for."
Flanker Sion Bennett has agreed a long-term contract with Cardiff Blues.