The Scottish Government agency said the tower deck had been recognised by Guinness World Records as the largest freestanding balanced cantilever in the world.
It said the bridge’s central deck is now complete but still free standing - and the record-breaking 644m cantilever would be a temporary phenomenon until the other sections were joined to it.
This section of the deck will be connected to the flanking towers and viaducts to form the completed bridge.
However, Transport Scotland said Guinness World Records experts had inspected and officially ratified the record while the deck was is in this "world-record breaking but still temporary state".
The news came as the agency repeated that the bridge would open next May, to replace the Forth Road Bridge.
Bridge officials said each deck section was added to ensure the cantilever remained balanced, which avoided placing too much stress on the concrete tower by adding deck segments sequentially at alternate ends.
They also pointed out that this construction method meant the ends of the deck could be temporarily up to 4m out of line in the deck itself, which had led some observers to question whether they would join up.
The balanced cantilever method has never been used to construct a bridge on this scale.
Economy secretary Keith Brown said: "We can all agree the Queensferry Crossing is a modern marvel and a world-class feat of engineering.
"It’s only fitting then that the bridge has been awarded a Guinness World Records title.
“This world-record breaking structure is all the more remarkable when you consider the extreme weather conditions often experienced out in the Firth of Forth, especially working up above the water between 60m and 210m high.
"Everyone who has worked so hard and skilfully to build this amazing bridge is a world record beater in their own right.
“It won’t be long before the balanced cantilever disappears, when the small gaps between the towers are closed. But the record is still there to be beaten and the Queensferry Crossing will still be the tallest bridge in the UK and longest bridge of its type in the world.”
Alan Platt, construction director for the Forth Crossing Bridge Constructors consortium (FCBC), said: "Construction work is going well on all three towers.
"This unique achievement at the centre tower is a feat of engineering which the whole FCBC team is immensely proud of.
"This is leading edge civil engineering and I’m delighted to pay tribute to the skills and dedication of everyone involved.”
The record-breaking cantilevered structure comprises 36 steel and concrete composite deck sections, which are some 40m wide, 16m long and 5m deep, and weigh an average of 750 tonnes.
Each one had to be lifted 60m from river level to be welded and bolted into position, with steel cables being simultaneously installed to bear their weight.