The Combat Hospital actress has appeared in five episodes of the CBS drama as Catherine Rollins, the love interest of Steve McGarrett (Alex O'Loughlin).
Borth will reprise the role full-time in the show's third season, Deadline reports.
Hawaii Five-0 was officially renewed by CBS earlier this month, along with 17 other shows.
Borth is the second recurring star to be promoted to series regular status. Last year, Masi Oka (Heroes) was also made a series regular following four appearances as medical examiner Max Bergman.
Hawaii Five-0 continues on Mondays at 10/9c on CBS. The show's second season will conclude on May 14.
In the UK, the series airs on Sky1 on Saturdays at 8pm.
> Hawaii Five-0 Alex O'Loughlin in rehab, taking hiatus
Watch a clip from the last episode of Hawaii Five-0 below: