Scientists have long debated whether water was here at the start or arrived later with comets and meteorites.
Now researchers from Glasgow and Hawaii Universities have found water trapped in some of the earliest-known rocks.
The believe Earth's water was likely carried here on dust that orbited the Sun before the planets formed.
The Glasgow and Hawaii team used "advanced ion microprobe technology" to examine rocks from Baffin Island, in the Canadian territory of Nunavut.
Dr Lydia Hallis led the research, first at the University of Hawaii at Manoa and then as a Marie Curie Research Fellow at the University of Glasgow.
She said the Baffin Island rocks had been analysed extensively since their collection in 1985 and scientists "know they contain a component from Earth's deep mantle".
"Essentially, they are some of the most primitive rocks we've ever found on the surface of the Earth, and so the water they contain gives us an invaluable insight into the Earth's early history and where its water came from," she said.
"We found that the water had very little deuterium, which strongly suggests that it was not carried to the Earth after it had formed and cooled.
"Instead, water molecules were likely carried on the dust that existed in a disk around our Sun before the planets formed. Over time this water-rich dust was slowly drawn together to form our planet.
"Even though a good deal of water would have been lost at the surface through evaporation in the heat of the formation process, enough survived to form the world's water."
Dr Hallis added: "It's an exciting discovery, and one which we simply didn't have the technology to make just a few years ago. We're looking forward to further research in this area in the future."
The team's findings are covered in the paper, "Evidence for primordial water in Earth's deep mantle", which has been published in the journal, Science.
New evidence has emerged which supports the theory that water was present on Earth when it formed from rock and dust more than 4.5 billion years ago.