Lahti will play the mother of Steve McGarrett (Alex O'Loughlin), Doris McGarett, who was thought to be dead, but took on the alias "Shelburne" and went into hiding.
"The mysterious character of Doris McGarrett is a pivotal one to the mythology of our show," executive producer Peter Lenkov said in a statement. "We are thrilled to have a seasoned and beloved actress such as Christine to portray her. She's a wonderful addition to the H50 family and the fans will certainly enjoy what we have in store for her. Let's just say that the apple doesn't fall far from the tree."