A gem trader said the stone was found by workmen digging a well in his home in the gem-rich Ratnapura area.
Experts say the stone, which is pale blue in colour, has an estimated value of up to $100 million in the international market.
The cluster weighs around 510 kilograms or 2.5 million carats and has been named the "Serendipity Sapphire".
"The person who was digging the well alerted us about some rare stones. Later we stumbled upon this huge specimen," Mr Gamage, the owner of the stone, told the BBC.
He did not want to give his full name or location for security reasons.
Mr Gamage, who is a third-generation gem trader, informed authorities about the find, but it took more than a year to clean the stone of mud and other impurities before they could analyse and certify it.
During the cleaning process, Mr Gamage said some stones fell out of the cluster and they were found to be high quality star sapphires.
Ratnapura, which means city of gems in Sinhalese, is known as the gem capital of the South Asian country. Other valuable stones have been found there in the past.
Sri Lanka is a leading exporter of sapphires and other precious gems.
Last year, the country earned around half a billion dollars through the export of gems, cut diamonds and jewellery.
"I have never seen such a large specimen before. This was probably formed around 400 million years ago," Dr Gamini Zoysa, a renowned gemmologist told the BBC.
Experts also point out however, that even though the specimen has a high carat value, all the stones inside the cluster may not be high-quality.
The find comes as Sri Lanka's gem industry has suffered losses as a result of pandemic induced lockdowns.
Those working in the industry hope the "Serendipity Stone" will now attract international buyers and experts.
"It is a special star sapphire specimen, probably the biggest in the world. Given the size and its value, we think it will interest private collectors or museums," Thilak Weerasinghe, the Chairman of the National Gem and Jewellery Authority of Sri Lanka, said.