The eye-catching piece of jewelry features a photo of Dolly on a charm.
Last July, Theroux took to Instagram to share candid photos from a ceremony held to say goodbye to the beloved pet.
"Tonight, at sunset, after a heroic struggle ... our most loyal family member and protector, Dolly A. laid down her sword and shield," the actor wrote alongside a collection of photos.
The memorial gallery included a photo of Dolly during happier days, as well as more intimate photos of Dolly at the end, stretched out on the grass with wildflowers and petals scattered across her bright white fur.
In one photo, two hands — which, based on a tattoo and a beauty mark, are believed to be Theroux's and Aniston's hands — grasp each other as they reach over Dolly. In another, Dolly's packmate, a terrier named Clyde, can be seen approaching her body.
In the final photo, the two aforementioned hands tenderly wrap Dolly in a soft blanket.
"She was surrounded by her entire family," Theroux wrote.
Aniston named Dolly after one of her idols, country music superstar Dolly Parton. The pair went on to share laughs about it last year while promoting their Netflix coming-of-age comedy, "Dumplin.'"
Aniston's new necklace isn't the first time she's honored a cherished canine. In 2011, the Emmy winner got a tattoo on her foot of her late dog Norman's name.