Lohan, 29, was seen adorning a pink gown with an emerald jewel on her ring finger. Two of them were attending the Caudwell Children's Butterfly Ball, which works to empower disabled children, reported E! online.
The "Freaky Friday" star also shared a picture from the event on Instagram.
"@clairelcaudwell and @johncaudwell thank you for including us to support all of the work you do for children, Bless you," she captioned the photo.
The couple have been together for about 10 months, maintaining a level of privacy while staying in London. However, Lohan recently opened up about Tarabasov, the special man in her life.
The actress moved to London after her probation ended in May 2015 and was recently seen spending quality time in a tropical beach city with Tarabasov.