Ambani's net worth has soared past $50 billion, making him the first Indian and only the fourth person in the world to have a wealth higher than this amount.
The RIL chief is now believed to be next only to software czar Bill Gates of the US, Mexican business baron Carlos Slim Helu and Warren Buffett, regarded as the world's greatest investor.
Based on the closing share prices of various group companies such as RIL, Reliance Petroleum, IPCL and Reliance Industrial Infrastructure, Mukesh Ambani is estimated to hold shares worth $50.1 billion (about Rs 2,00,000 crore) through promoter holdings in these companies.
The four companies together have a market value of Rs 4,09,325 crore ($103 billion).
On the other hand, Mittal owns shares worth about $48.4 billion in ArcelorMittal, the world's biggest steelmaker in terms of revenue, assets and market value.
Shares of ArcelorMittal, in which Mittal family holds 44.79 per cent, were trading around 55 euros ($77) in European market today, giving it a market cap of about $108 billion.
While the net worth of Ambani and Mittal are based on the current market values of their group companies, that of Gates, Buffett and Carlos Slim are based on figures for August-end.