The 47-year-old star of That ‘70s Show was sentenced to 30 years to life in prison for the rapes of two women two decades ago.
Kunis and Kutcher, who married in 2015, both costarred in the American sitcom with Masterson, which ran for eight seasons and 200 episodes between 1998 and 2006.
In their letters of support, published by Los Angeles-based court reporter Meghann Cuniff on Friday, the couple vouched for Masterson’s “exceptional character” and pleaded for Judge Charlaine Olmedo’s leniency.
Kutcher called his co-star a “role model” and “a person that is consistently there for you when you need him”.
“While I’m aware that the judgement has been cast as guilty on two counts of rape by force and the victims have a great desire for justice, I hope that my testament to his character is taken into consideration in sentencing,” his letter reads.
“I do not believe he is an ongoing harm to society and having his daughter raised without a present father would a tertiary injustice in and of itself. Thank you for taking the time to read this.”
Meanwhile, in her letter, Kunis called Masterson “an amazing friend, confidant, and, above all, an outstanding older brother figure to me”.
“Danny Masterson’s warmth, humor, and positive outlook on life have been a driving force in shaping my character and the way I approach life’s challenges,” she wrote.
“I wholeheartedly vouch for Danny Masterson’s exceptional character and the tremendous positive influence he has had on me and the people around him. His dedication to leading a drug-free life and the genuine care he extends to others make him an outstanding role model and friend.”
The Independent has contacted Kunis’ and Kutcher’s representatives for comment.
Actress Debra Jo Rupp and actor Kurtwood Smith, who played Kitty and Red Forman on That ‘70s Show, also wrote letters, according to Cuniff.
Kutcher, who also starred with Masterson in Netflix’s The Ranch, spoke publicly about the trial for the first time in January this year.
“Someday, his kid is going to read about this,” Kutcher said of the trial’s revelations in an interview with Esquire.
He stipulated: “I wholesale feel for anybody who feels like they were violated in any way.”
“Mr Masterson, you are not the victim here. Your actions 20 years ago took away another person’s choice and voice. Your actions 20 years ago today were criminal, and that’s why you are here,” Judge Olmedo said while handing down Masterson’s sentence on Thursday.
Masterson had initially been accused of drugging and raping three women at his home between 2001 and 2003. He was convicted of raping two of the women in 2003, but the jury was unable to reach a verdict on the third allegation from November 2001, brought forward by a former girlfriend.
An old 2004 chat show clip featuring Masterson, which takes on chilling new light in the wake of his conviction, resurfaced on social media following his sentencing.