The project is part of the company's plans to stop using fresh water altogether in Chile from 2030.
The new 2,500l/s plant is in addition to the existing 525l/s plant which has been operating for 12 years.
BHP said the investment covered two pipelines that took the water 3,200m above sea level, four high-pressure pumping stations, a reservoir at the mine and high-voltage electricity infrastructure to operate the system.
"At present, we are operating three concentrators simultaneously, an unprecedented situation in the mining world, giving us a greater processing capacity with which to maintain our copper output," Minera Escondida president Mauro Neves said.
"In this, the new desalination plant plays a crucial role."
BHP minerals Americas president Danny Malchuk said the inauguration of the largest desalination plant in Latin America was an historic achievement.
The mine produced 772,000 tonnes of copper in FY17 for BHP and the company has put FY18 guidance at 1.13-1.23 million tonnes, with volumes weighted to the second half reflecting the full utilisation of the three concentrators.
The Escondida joint venture, 57.5% owned by BHP and 30% by Rio Tinto (LN:RIO), is 3,100m above sea level in Antofagasta.
Early labour talks began last week between Escondida and the site's main union as workers' existing contracts are due to expire mid-year.
