The extra £1m is in addition to an earlier grant of £4.3m and will allow the Western Isles Council to start work on the project this financial year.
Mr Forsyth said: 'I am very pleased to announce this substantial additional government funding to enable the construction of the Berneray Causeway. The funding package includes extra capital allocations of £1m which will be issued to the council for the project. This is on top of the £4.3m grant which I authorised earlier this year.
'The building of a causeway between the Islands of Berneray and North Uist has been a top priority of the local community, and of the Western Isles Council, for sometime now. They have fought hard to get this project off the ground and I am delighted that I have been able to help.