The decision was announced at a meeting in the German city of Bonn after the UN's cultural committee spent more than a year considering its nomination.
World heritage status is given to sites of "outstanding universal value" with the aim of protecting them for future generations.
The distinctive red bridge has carried trains over the Forth since 1890.
Scotland's other World Heritage Sites are New Lanark, St Kilda, the Old and New Towns in Edinburgh, Neolithic Orkney and the Antonine Wall.
The award puts it alongside the Pyramids of Egypt, the great Wall of China and the Sydney Opera House in terms of cultural significance.
The bridge, which spans the Firth of Forth between South Queensferry on the outskirts of Edinburgh and North Queensferry in Fife, was opened in 1890 after eight years of construction.
Designed by Sir John Fowler and Benjamin Baker, it measures 2,529m (1.5 miles), weighs 53,000 tonnes and was at the time the world's longest multi-span cantilever bridge.
When it was constructed it was one of the most ambitious projects of its kind ever attempted, and at its peak, more than 4,500 men were employed building it.
The Unesco inspection report stated: "This enormous structure, with its distinctive industrial aesthetic and striking red colour, was conceived and built using advanced civil engineering design principles and construction methods.
"Innovative in design, materials and scale, the Forth Bridge is an extraordinary and impressive milestone in bridge design and construction during the period when railways came to dominate long-distance land travel."
For 125 years it has been an icon of Victorian engineering excellence, a symbol of Scotland and even a favourite expression for a never-ending task.
Now the Forth Bridge is listed alongside the Pyramids of Egypt, the great Wall of China and the Sydney Opera House in terms of cultural significance.
We've brought together some facts and figures - and more great pictures - of one of Britain's best-known structures, which you can see here.
The bid for World Heritage status was led by the Forth Bridges Forum, which was established by the Scottish government to promote the three Forth Bridges.
Scottish First Minister Nicola Sturgeon said the bridge was one of the "industrial wonders of the world" and congratulated the team behind the bid.
She added: "The Forth Bridge is an outstanding example of Scotland's built heritage.
"Its endurance is testament not only to the ingenuity of those who designed and built it but also to the generations of painters, engineers and maintenance crews who have looked after it through the years."
The bridge is owned by Network Rail, whose infrastructure director, David Dickson, described it as "a prime example of civil engineering and an iconic structure, not only in Scotland but across the world."
Mike Cantlay, chairman ofTourism agency VisitScotland said World Heritage Site status would lends "even greater aura and appeal to one of the planet's most instantly recognisable landmarks".
He added: "The timing is perfect as, in 2016, this country will celebrate the Year of Innovation, Architecture and Design and you would be hard-pushed to find a better example of all three qualities anywhere in the world than in the Forth Bridge."
UK Heritage Minister Tracey Crouch said: "Recognition as a World Heritage Site will draw more tourists to the area as well as making sure one of the UK's great engineering feats stands for future generations."
There are now more than 1,000 World Heritage Sites across the globe, in 161 countries.
Of these, 29 are British, including the Tower of London, the Giant's Causeway and Stonehenge.
The Forth Bridge has become the sixth Scottish landmark to be awarded Unesco World Heritage Site status.