The NBA star has taken over as his brother's primary caregiver in the wake of their mother's sudden death.
The 32-year-old NBA star has been serving as Amari's primary caregiver in the wake of Andrea's death from a heart attack in January.
In court documents obtained by ET, Tristan received the temporary custody of Amari on Wednesday by the Superior Court of California.
This decision gives the Los Angeles Lakers star "all powers a parent having legal custody of a child would have." The temporary guardianship is granted through Dec. 5, 2023.
Amari suffers from severe epilepsy and is wheelchair bound. He requires round-the-clock care.
In the documents, Tristan says he does not know where his father, Trevor Douglas Thompson, is at this time. He also plans to file paperwork to help make Amari a U.S. citizen as he was previously residing and receiving treatment in Canada where he lived prior to Andrea's death.
In the season 3 finale of The Kardashians, Tristan's ex, Khloé Kardashian, allowed the athlete and Amari to temporarily move into her Calabasas, California, home after Tristan's home suffered flood damage.
"Tristan and I are not back together. I know it's hard for everyone to believe. But I love, love, love Andrea. I love Tristan. I love Amari," Khloé explained on the show. "This is what family does. This is the father of my kids. I lost my dad [Robert Kardashian] and my dad was like a fairy-tale parent, but still I can't wrap my head around losing my mom and I know how close Tristan and his mom are and it's just heartbreaking. And then to be left with the responsibility of another person as well. It's a lot… Tristan and I are family and we're gonna be family for the rest of our lives."
Both Khloé and Tristan also celebrated Amari's 17th birthday back in July. Khloé threw Amari a party for the special occasion.
"Someone is 17 today!!!! Happy birthday sweet sweet Amari!!! We are all so blessed to have been touched by an angel such as you you are truly one of Gods greatest treasures " Khloé captioned pics of herself, mom Kris Jenner, and her and Tristan's daughter, True Thompson.
Tristan also celebrated the occasion, writing, "Happy birthday to my sweet baby brother Amari! I love you forever! We Miss you mommy ❤️."