"Donatella Versace is making my dress as we speak," the pop star announced on Tuesday on Instagram, along with pictures of her dressed in a pink gown.
Versace and Spears are old friends. In May, the designer posted a throwback photo of them together from her brand's show in 2002.
"This flashback is from when she came to my show in 2002, looking glamorous in Versace. I’ll never forget when you came and stayed with me Britney, you were and will always be magical," she said.
In October, Spears and Asghari asked fans for suggestions on a location for their nuptials.
"We're in a predicament right now. We have a really big problem. I don't know where I'm going to get married. We don't know whether we're going to get married in Greece or Italy or Australia or New York City," she says in the video, posted while on holiday in French Polynesia.
"Why don't you ask the fans," Asghari chimes in, to which Spears replies: "That's a very good idea."
The record-breaking singer has been on a high after the conservatorship that has controlled her life for 13 years neared a possible end in September, when a judge suspended her father as the conservator. Spears later posted on Instagram that she was "on cloud 9 right now".
A hearing is scheduled for this Friday to discuss a petition to terminate the conservatorship.
Spears and Asghari announced their engagement early in September following a five-year courtship. The couple met in 2016 on the set of Spears's Slumber Party video, in which Asghari plays the singer's love interest.
The engagement ring, designed by Roman Malayev at Forever Diamonds NY, is estimated to have cost $70,000 and has the word "lioness" engraved inside, which is Asghari's nickname for Spears.
"Our team couldn’t be more thrilled to congratulate Britney Spears and Sam Asghari on the beginning of their forever," a post on the Forever Diamonds NY Instagram account reads.
"The thought and detail Sam put into this ring made the design process so special for Roman [Malayev] and our team. In honour of the lioness herself, we are naming this gorgeous setting The Britney. Sam and Britney, we wish you a lifetime of happiness together."
Spears has been married twice, first to childhood friend Jason Alexander in January 2004, but the Las Vegas marriage was annulled after 55 hours. She then married Kevin Federline in September 2004. The couple divorced in July 2007 and have two sons, Sean Preston, 15, and Jayden James, 14.