SEOUL (Reuters) - A gauge of South Korean manufacturers' sentiment over future business conditions rose to its highest in a year-and-a-half for September, a central bank survey showed on Wednesday, as exports prospects brightened from the previous month.

The Bank of Korea's manufacturing business survey index (BSI) for September stood at a seasonally adjusted 77, compared with 73 for August.

It was the highest reading since the index stood at 80 in March 2015.

An index reading below 100 indicates that the number of companies that expect business conditions to deteriorate in the following month outnumbers those seeing improvement.

The index has remained below 100 since January 2011.

The outlook for future business sentiment rose for all respondents, especially for exporters and conglomerates.

A sub-index showed sentiment improving over a majority of indexes including exports, production and production equipment.

On Tuesday, a Reuters poll showed exports in August are expected to rebound into positive territory for the first time in 19 months, mainly aided by smartphone exports and two more working days this year.

(Reporting by Christine Kim; Editing by Subhranshu Sahu)