Scot Gemmill opened the scoring for Nottingham Forest in the 15th minute with a right foot volley from the right of the penalty area.
Southampton pulled one back in the 64th minute with Matt Le Tissier header from six yards out after a cross from the left.
The match went to extra time and Scot Gemmill hit the winning goal and his second with five minutes remaining when he volleyed in from six yards after a cross from the right.
Des Walker collected the trophy, having taken over as captain when Stuart Pearce was substituted with an injury in the first half.
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