With Long's opener against Watford, his third in four games, he bettered the previous record-holder, Ledley King, by some margin.
In Shane Long's case on Tuesday, he needed just 7.69 seconds, as he charged down a clearance from Watford's Craig Cathcart before giving Southampton a record-breaking lead at Vicarage Road in their English Premier League match.
"It's nice", Long told Saints' official website.
Long's historic goal, just his fourth this season, didn't prove to be the victor at Vicarage Road after Gray's 90th minute equaliser for the FA Cup finalists but nonetheless moved them six points from the relegation zone with three matches remaining.
The Republic of Ireland global closed down Craig Cathcart's attempted clearance and powered through to clip over Ben Foster after just seven seconds.
King found the net against Bradford in 2000 after just 9.82 seconds and held the record for the best part of two decades.
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Guardiola's men need to bag in all three points to be able to leapfrog title rivals Liverpool and move to the top of the table.
Opta had the goal at six seconds.
Christian Eriksen scored after 10.54 seconds for Tottenham against Manchester United last season.
And Long's goal helped Southampton to another vital point in their quest for safety.
It's taken 19 years but the Premier League has a new fastest goal record..
A straightforward pass from the kick-off by Roberto Pereyra to Craig Cathcart ended with the defender striking the ball against Shane Long, who skipped forward to chip over the advancing Ben Foster.
Watford moved above Everton into seventh place to keep up their hopes of qualifying for the Europa League.