"I dedicate this to the brave survivor in the Stanford case who has given so much to change the conversation," Dunham wrote.
Turner, 20, was found guilty of sexually assaulting an intoxicated and unconscious woman behind a trash bin on campus in 2015.
He was sentenced last week to six months in county jail and then three years of probation, and he was ordered to register as a sex offender — a perceived light sentence that ignited an outcry from his victim and others.
In the PSA, Dunham, along with Allison Williams, Jemima Kirke and Zosia Mamet, reiterated the statistics on sexual assault.
The Centers for Disease Control and Prevention state that in the United States, nearly one in five women, and one in 71 men, have reported being raped at some point during their lives; about one in 20 men and women reported other forms of sexual violence.
"And in 80 percent of those cases, those attacks are perpetrated by someone they already know," Williams said in the video.
"This isn't a secret; it's reality," the four women took turns saying. "So why is our default reaction as a society to disbelieve, or to silence, or to shame? What if we chose to turn towards those in need, instead of away? To listen? To support?
"You have the choice to make things better."
At Turner's sentencing last week, his 23-year-old victim read from her emotional and powerful impact statement, saying, "You took away my worth, my privacy, my energy, my time, my intimacy, my confidence, my own voice, until today."
Then she spoke to other survivors of sexual assault:
And finally, to girls everywhere, I am with you. On nights when you feel alone, I am with you. When people doubt you or dismiss you, I am with you. I fought everyday for you. So never stop fighting, I believe you. Lighthouses don't go running all over an island looking for boats to save; they just stand there shining. Although I can't save every boat, I hope that by speaking today, you absorbed a small amount of light, a small knowing that you can't be silenced, a small satisfaction that justice was served, a small assurance that we are getting somewhere, and a big, big knowing that you are important, unquestionably, you are untouchable, you are beautiful, you are to be valued, respected, undeniably, every minute of every day, you are powerful and nobody can take that away from you. To girls everywhere, I am with you. Thank you.
"When I finally chose to share my story, I did not do so in a vacuum," she wrote in a 2014 op-ed for BuzzFeed News. "I was inspired by all the brave women who are now coming forward with their own experiences, despite the many risks associated with speaking out. Survivors are so often re-victimized by a system that demands they prove their purity and innocence. They are asked to provide an unassailable narrative when the event itself is hazy, fragmented, and unspeakable. They are isolated and betrayed by people close to them who doubt their reality or are frustrated by their inability to move on. Their most intimate experiences are made public property."
She added: "I hoped I might inspire others to share, and that forming these connections would assist us all in healing."
The women said in their recent PSA that standing up against sexual assault is one issue that they all four agree on.
"You already have the power to create a safer, healthier environment for women to come forward," they said. "And while it's just the four of us here right now, we hope to represent the solidarity and support all survivors should be able to find."