James Flude, 28, said he discovered the silver ring with diamond-like stones on the northbound platform at Vauxhall London Underground station on Saturday night.
He posted a photograph of the ring on social media after trying to hand it to station staff, but found them “pretty unhelpful”.
On his Twitter page, Flude wrote: “I found this engagement ring at Vauxhall station at about 22:30 last night. Would love to get it back to its owner and station staff were pretty unhelpful in my quest.”
The tweet has garnered hundreds of likes and retweets, and many people wished him luck in his hunt for the owner.
“We have a legal obligation to keep items found on our network in safe custody, allowing owners to process a claim,” added the transport network. “Thanks for your honesty.”
However, according to the TfL website, lost property is kept for a maximum of three months from the date of loss. After that, it becomes the property of TfL and may be donated to charity, recycled, disposed of or sold.
Flude told The Independent that he has had “no real joy yet”, and that one person has tried to claim the jewellery “dishonestly”.
He added that he had “a couple of leads which didn’t come to fruition”.
According to Flude, who is from Shepherd’s Bush, he came upon the ring “glistening” on the ground after watching The Hundred cricket at the Oval. He was at Vauxhall staton at the time because he lost his earphones and had gone back to the pub to find them.
Speaking to the BBC, he described the discovery as “so weird” because he had just lost something as well.
“There’s nothing worse than losing something of great value, whether it’s financial or emotional, so it’d be great to reunite them with the ring,” he said.