The 15-year-old, who wore a black tuxedo and a buzz cut, introduced themselves to the gathering.
Hollywood star Ben Affleck and his ex-wife actor Jennifer Garner's 15-year-old child has come out as transgender on Wednesday. At a memorial service for Ms Garner's father, they announced their new name, Fin Affleck.
The 15-year-old, who wore a black tuxedo and a buzz cut, introduced themselves to the gathering. "Hello, my name is Fin Affleck," they said, before reciting a verse from the Bible at the memorial service.
While Fin was spotted with the new haircut earlier this year, they came out publicly as trans and revealed their new name only on Wednesday.
Mr Affleck and Ms Garner split in 2015 and the actors share custody of their three children: Violet Anne, 19 and Fin Affleck, 15 and Samuel Garner, 12.
Both Mr Affleck and Ms Garner have been outspoken about keeping their children out of the limelight, with Mr Affleck also calling for law that mandates media outlets to blur children's faces in published photos on the lines of a similar legislation in the UK. "You can say what you want about me. You can yell at me with a video camera and be TMZ. You can follow me around and take pictures all you want. I don't care. That's part of the deal. But it's wrong and disgusting to follow children around and take their picture and sell it for money. It makes the kids less safe," he told a news outlet in 2013.
Ms Garner shared the news of her father's death on Instagram along with several photos of her family over the years. "My dad passed peacefully Saturday afternoon. We were with him, singing Amazing Grace as he left us (did we carry him across or scare him away- valid question.) While there is no tragedy in the death of an 85 year old man who lived a healthy, wonderful life, I know grief is unavoidable, waiting around unexpected corners," she wrote.
The 51-year-old star said the post was intended to share her memories with her father and show her "appreciation for the kind and brilliant man, father, and grandfather he was, as well as the loving legacy he left behind."