In a written statement shared on Thursday on the photo-sharing app, Jenner, 28, tells fans that he's spent time considering how to address the allegations he said were made in "late-2019."
The actor said he'd previously kept a part of his life "in darkness out of shame and fear."
"When I was 20 years old, I met a woman and we fell in love," he wrote. "I didn't realize the magnitude of it back then, but in retrospect, as great as the love that was shared between us, the shared brokenness that stemmed from our childhoods proved to be greater."
Jenner said he and his partner experienced "joyous highs, but also moments of jealousy, bouts of insecurity, and volatility," but he felt as though he had no choice but to stay in the relationship.
"I take full responsibility and accountability for the hurt that I inflicted during my relationship with my past partner — emotionally, mentally, and yes, physically," Jenner admitted.
The star then recounted an incident in which he and his partner were arguing when, "in a moment of frustration," he threw his phone "aimlessly" and hit his partner in the face.
"I froze in a state of shock and horror as my then-partner screamed in anguish, her eye immediately swollen shut from the impact of the phone," he recounted. "It's a moment that I will regret for the rest of my life."
Thought Jenner never mentions anyone's name, the allegations match up with a statement made by Jenner's ex-wife Melissa Benoist late last year in which she described a similar incident. She also said in her statement that she also became violent in self-defense and never revealed the name of her alleged abuser.
The two co-starred in "Glee" together and were married from 2015-2017.
Reps for Benoist did not immediately respond to Fox News' request for comment.
Jenner continued and said that "without absolving myself of any responsibility, it’s important to understand that there was mental, emotional and physical abuse inflicted on both ends."
The attended therapy together, but ultimately, the counseling was unsuccessful, said the actor.
The "Glee" alum said that he has gone to therapy for "years" and has come to realize that his "traumas" were a result of a "false sense of masculinity that I felt the need to uphold."
Blake Jenner admitted to physically, emotionally and mentally abusing an ex. (Getty Images)
Jenner insisted that his explanation is not meant to serve as an excuse.
"I also do believe that when allegations and information are brought forth about someone, that accountability goes both ways and one has a right to defend oneself when deemed necessary," the actor wrote.
Jenner continued, saying he was made to give up job opportunities out of jealousy of female co-stars and was threatened against taking photos with female co-stars. Additionally, he claimed his former partner would call him while he was away with threats of self-harm.
"I was verbally and emotionally abused for everything from the family I was born into to the clothes I would wear in photoshoots," Jenner said. "I was scratched. I was slapped. I was punched in the face, which caused a trip to the hospital to treat my broken nose."
Additionally, the star said he was "assaulted in the shower" resulting in a "traumatic injury" that he didn't wish to discuss further.
The actor said there were "discrepancies between the recollections of our relationship," but said that he would not air out such laundry out of respect for his former partner.
Jenner said he didn't "wish to drag her down in any way," but wants for both himself and his former partner to "heal."
The actor continued his statement by offering up a handful of apologies, the first to those that have lost respect for him and another to victims of "situational couple violence," which was accompanied by a promise to "lead with action."
"I stand with you as a victim myself, I repent for the abuse I inflicted as an aggressor myself, and vow to never retreat to losing who I am ever again," he said before addressing "the most important apology" he had to share: One to his former partner.
"I am sorry. There are many things that I wish I could have done differently; and while I wish it did not take the pain that was suffered throughout the course of our relationship, I will never regress to making the same mistakes ever again," Jenner stated. "I will never stop doing the work necessary to better myself in all areas of my life. I wish you and your family nothing but good health, joy and love."
Benoist, 31, is now married to actor Chris Wood.
Jenner concluded his statement by thanking fans for reading his "truth."