The Redbridge viaduct between Tweedbank and Galashiels has been closed so that work, linked with the Borders Railway, can be carried out.
A popular footpath which runs over the bridge will also be closed, meaning a diversion of a mile for the people who use it.
Hugh Wark from Network Rail said:"We know that the Redbridge Viaduct link between Langlee and Tweedbank is an extremely popular walking and cycling route and we've done all we can to retain that access until now.
"The improvements we're making to the bridge mean that this relatively short eight week closure is unavoidable - we apologise for the temporary inconvenience.
He continued: "The bridge itself is in excellent condition and the structural works required are minimal. Once complete, this grand old structure will form another iconic landmark on the Borders Railway route."