Mexican telecommunications magnate Carlos Slim remained the world's richest person last year holding a personal fortune of more than $70 billion, it said.
Ambani, 55, has also retained his position as the world's richest Indian for the sixth year in a row and increased his ranking from 19th to 18th position and personal wealth from $21 billion to $ 24.7 billion in the world, the index showed.
The world's top 100 billionaires got even richer in 2012, with their total wealth rising by around 15 per cent to $1.81 trillion, it said.
Microsoft founder Bill Gates and the founder of fashion retailer Zara, Amancio Ortega, rounded out the top three spots, with wealth estimated just over and just under $60 billion, respectively.
Renowned investor Warren Buffett slipped to fourth place, but added around $5 billion to his fortune over the year despite donating a substantial amount to charity last year.
IKEA founder Ingvar Kamprad saw his net worth rising 16.6 per cent to approximately $40 billion, putting him in fifth place the index.