He was writing on Twitter after officials from Qatar and India signed seven agreements covering areas such as investment, financial intelligence and education.
"My visit to Qatar has witnessed comprehensive and fruitful discussions that will usher in a new era of friendship between India and Qatar," Modi said.
Included was a memorandum of understanding to open up the Indian market to Qatar's sovereign wealth fund, the estimated $300bn Qatar Investment Authority.
The memorandum said it aimed to facilitate the "participation of Qatari institutional investors in infrastructure projects in India".
The Indian community is the largest ethnic group in Qatar, numbering some 700,000 in a total population of just over 2.5mn.
Qatar is already India's largest supplier of liquified natural gas, and bilateral trade is thought to stand at around $10bn.
Earlier on Sunday, Modi met HH the Emir, Sheikh Tamim bin Hamad al-Thani.
Modi will also visit Switzerland, Mexico and the United States.
