The woman met her victim, a 43-year-old Manchester man, on Match.com before convincing him to hand over vast sums of cash.
She reportedly spoke with an American accent and claimed to be a 35-year-old from the Bahamas who worked in South Africa, but police do not know her real identity or age.
The brunette is said to use three different names – Christy Dunn, Martha Cameron and Miranda Stinson – and is believed to have links to the Bahamas, South Africa, Botswana, London and Bristol.
Avon and Somerset police issued a photograph of the woman ahead of a Crimewatch appeal broadcast last night.
Detective constable Simon Da Costa said the woman met her victim in February 2014, communicating with him through the website’s direct messaging facility and then with video calls via Skype.
“Within ten months of them meeting online, the offender made multiple requests to the victim for money," he said.
“The victim was manipulated into giving this money firstly by the promise of a relationship, then emotional blackmail and the promise of the return of his money, and finally by threats.
“We really need to find this woman, so if you have information about where she may be, or if you have also been a victim, please call us.”
Some of the money was paid into accounts with links to Bristol, but police believe the scam is likely to be part of a wider criminal operation targeting lonely and vulnerable people online.
So-called “romance fraud” has increased dramatically with the rise of online dating, and fraudsters are believed to have stolen nearly £40m from Britons looking for love last year, according to official crime figures.
An estimated 8 million adults in the UK used online dating sites in 2016 – a huge increase from just 100,000 at the turn of the century.
On average, victims of romance scams are conned out of around £10,000.
Overseas criminals posing as suitors targeted almost 4,000 wealthy Brits in 2016 according to the National Fraud Intelligence Bureau.
Both men and women are targeted, with at least 39 per cent of those duped thought to be men.
Investigators fear these statistics could be the tip of the iceberg as many victims are too embarrassed to tell their family and friends they have been swindled and do not report the scams to the police.
All dating websites warn users not to send money to people they have not met, but cyber psychologist Monica Whitty of the University of Leicester said: “It is not the case that stupid people fall for romance scams – they can be very clever.”
Match.com gives detailed advice to its users on how to spot the signs of a potential fraudster.
The website states: “Our customer care team works hard to ensure that unwanted accounts are removed from the site. Our moderation team manually check photos and personal ads across the site and a built-in screening system helps identify suspicious accounts, remove them and prevent re-registration.
"While we are confident that our measures ensure a high level of security, we urge members to maintain vigilance while dating online and report any suspicious profiles to safeguard other members.”