The Union Chain Bridge, which dates from 1820 and spans the River Tweed four miles upsteam of Berwick-upon-Tweed, was dismantled a year ago as part of a £10.5m overhaul.
Engineers have now repaired all its components and strung the first of its 13 chains across the water.
Once the remaining chains are in place, deck hangers will be reinstalled and the bridge deck rebuilt, ahead of an expected reopening later this year.
The 200-year-old structure is thought to be the oldest vehicle suspension bridge in the world still in operation and was a major draw for span enthusiasts and other visitors to the area..
A funding package for repairs, which followed serious concerns about its condition, was put together by local authorities on both sides of the border, Museums Northumberland and the community group Friends of the Union Chain Bridge.
Northumberland County Council cabinet member for local services, John Riddle, said: "This is yet another milestone, and a very welcome one, to finally see this famous structure start to be put back together.
"We've worked hard to retain as many of the original parts as is possible while also ensuring the bridge is able to fulfil its main purpose, and everyone involved with the project is delighted to see the bridge once again taking shape."
Gordon Edgar, Scottish Borders Council's executive member for infrastructure, travel and transport, said it was "a hugely important project" which would protect "this historic structure for many, many years to come".
The Union Chain Bridge was built by retired naval captain Samuel Brown and completed in 1820.
It cost about £7,700 to construct and replaced a "perilous ford" further downstream.
Although work on Thomas Telford's Menai Bridge in Wales started before it, the Union Bridge was completed first, making it the longest iron suspension bridge in the world when it opened.
Hundreds of spectators - including civil engineers Robert Stevenson and John Rennie - turned out to see it open on 26 July.
Until the 1970s it existed with little maintenance but the entire deck was replaced in 1974.
Concerns about its condition have increased in recent years, finally prompting the complete overhaul.