The married pair wrote character references for their shamed co-star, who has been jailed for 30 years to life.
In an online video Kutcher, 45, said they did not intend to undermine the victims’ testimony or retraumatise them.
He added: “We would never want to do that and we’re sorry if that’s taken place.”
The couple appeared with Masterson, 47, in sitcom That ’70s Show around the time of the attacks in 2003.
Kutcher had told the rape trial judge in Los Angeles that he was a man who treated people “with decency, equality, and generosity”.
Kunis, 40, called him “an “exceptional older brother figure”.
But in the video she insisted: “We support victims.”
Masterson also starred alongside Topher Grace and Laura Prepon in That ‘70s Show from 1998 until 2006.
Grace, Prepon, Kunis and Kutcher have since reprised their roles with appearances in Netflix series That ‘90s Show, released in 2023.
Kutcher also starred opposite Masterson on American sitcom The Ranch, until the latter was written out of the show when an LAPD investigation was revealed in 2017.
On Thursday, before Masterson was sentenced, Los Angeles Superior Court heard statements from the women attacked by the 47-year-old in which they detailed their trauma and pain.
A jury of seven women and five men has found Masterson guilty of two counts on May 31 after seven days of deliberations.