The 26-tonne weight limit, due to come into force next Monday, March 20, will affect only heavy lorries.
The limit is a precautionary measure designed to reduce strain on the 250-year-old bridge, a grade-B listed structure, while preparations get under way for the forthcoming £640,000 repair scheme.
An inspection of the bridge in June last year revealed deterioration of its condition, necessitating repair and strengthening works.
Temporary signs will be in place to advise affected drivers of the limit and of the alternative route via the A6091, A7 through Galashiels and B6374.
The repairs, expected to to take place this summer and autumn, are part of a series of major works being carried out on Borders bridges this year and next.
That £2m programme of works also includes improvements to Clackmae Bridge near Earlston, Bowanhill Bridge near Teviothead and Bowden Church Bridge, west of Newtown.
The repairs to Lowood Bridge will cost £255,000 this coming financial year and £385,000 the year after.
A Scottish Borders Council spokesperson said: “The removal of the heaviest vehicles from Lowood Bridge is a precaution, with the bridge safe to use for remaining traffic. Once the repairs are completed, the intention is to remove the weight restriction.
“The increased investment in bridge repairs across the Borders will ensure these vital structures which link up our road network can be used for years to come.”