It said the deal, which will begin from April next year, will double its current customer base and provide a “springboard for growth”.
Its customers will include a mixture of businesses and public-sector organisations.
The move comes ahead of a shake-up of the English retail water market in April 2017 when businesses and public-sector bodies will be able to choose their water supplier.
Business Stream chief executive Jo Dow said: ” The new market will create huge opportunities for Business Stream. We are ideally placed to secure market share by leveraging our knowledge, capability and experience, gained from eight years of operating in the highly competitive Scottish market.”
Southern Water holds around 105,000 non-domestic customers throughout Kent, Sussex, Hampshire and the Isle of Wight.
Ms Dow added: “By creating the third largest UK non-domestic water supplier, we have also forged a bright future for our existing workforce, creating new jobs and building a foundation for growth when the English market opens fully.”
Business Stream said the deal will also create 40 new jobs at is headquarters in Edinburgh. The publicly -owned firm’s current English business customers include House of Fraser and Bernard Matthews.
Matthew Wright, chief executive of Southern Water, said: “After looking carefully at the options available to us we have decided to sell our non-household retail business to Business Stream, a specialist retailer that has been at the forefront of the competitive market in Scotland for eight years.”
Roseanna Cunningham, Scottish cabinet secretary for environment, climate change and land reform, said: “We are fully committed to keeping Business Stream in public ownership but it is right that Business Stream takes every opportunity to compete in the commercial marketplace in which it operates.
“The fact that this will create up to 40 new jobs and bring new revenue into the Scottish economy highlights the significant benefits of this approach.”