Leiua began his career at the Hurricanes prior to England move that saw him plying his trade at Wasps and the Bristol Bears.
The 33-year-old has 31 caps for his national team Samoa and will join his teammates when his work permit has been approved.
DHL Stormers Head Coach John Dobson believes the experience of Leiua will be of help to the young players of DHL Stormers.
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“Alapati is a proven performer who will give us a hard edge in midfield while also mentoring some of the talented youngsters we have here,” he told the DHL Stormers’ website
“We saw the huge influence Jamie Roberts had in a short time here two years ago and we are expecting Alapati to play a similar role for us this season.
“We are looking forward to seeing the impact he makes here both on and off the field.”
Leiua said: “We all saw this team play some incredible rugby last season and I want to add to that with my skills.
“This is a great chance to work with some talented young players and experience a completely new rugby culture, so I can’t wait to get stuck in.”