Lord of the Rings star Sean Astin will play Roland, an Alpha with the ability to absorb memories, TVLine reports.
NCIS actress Lauren Holly will also appear on the Syfy drama as a senator with a hidden agenda.
Summer Glau - who appeared in the show's first season as Skylar Adams - will additionally reprise her role for three season two episodes.
Southland's C Thomas Howell has also been lined up for a guest role, playing a man with super speed. Meanwhile, Caprica's John Pyper-Ferguson will reprise his role as Stanton Parrish, and CSI: NY's Kathleen Munroe will reappear as Dr Rosen's (David Strathairn) daughter Dani.
Former Eureka co-executive producer Matt Hastings has joined Alphas for the new season.
The series returns to Syfy on Monday, July 23 at 10/9c. In the UK, the series airs on 5*.
