The Dutch company has revealed neither the cost nor the expected date of the buy-out.
Seventh Generation's turnover exceeded US$200 million (€180 million) in 2015, Unilever said, adding that "it has seen double digit compounded annual growth over the last 10 years."
The US-based company sells recycled tissues, plant-based detergents, synthetic fragrance-free soaps and coconut oil-based products for babies.
"This addition to Unilever's product portfolio will help us meet rising demand for high-quality products with a purpose," said Nitin Paranjpe, who heads Unilever's home care business.
Active in more than 190 countries with a stable of household brands, including Lipton tea, Knorr soups, Rexona deodorants and Bertolli olive oils, Unilever employs some 169,000 people across the world.