The pop star posted a video to Instagram showing off a large diamond ring with a smiling Asghari by her side, with the caption "I can't (expletive) believe it !!!!!"
As Spears has battled to break free of the two conservatorships controlling her finances and personal life, she has claimed that they have not allowed her to marry or have a baby. That claim has been denied by Jodi Montgomery, the conservator of her person.
Spears' father Jamie controls her financial affairs but amid a lengthy legal battle he recently filed to remove himself from that role.
Britney Spears' father agrees to step down from conservatorship after months of public pressure
Asghari, a 27-year-old personal trainer, has been with the 39-year-old Spears for about five years.
Spears has been married twice before and has two children. She has talked publicly about wanting to marry again and have more children.
Asghari also posted a picture of the ring on his Instagram account.