Leif Davis gave the Blues the lead but Southampton but Che Adams and Adam Armstrong hit back for the Saints to give the visitors a 2-1 half-time lead at Portman Road.
Sub Jeremy Sarmiento scored a 97th-minute winner as Town came from behind to beat 10-man Southampton 3-2 in a Portman Road classic which will live long in the memory and return to the top of the Championship. Leif Davis gave the Blues the lead on 13 but Southampton hit back through Che Adams only a minute later and Adam Armstrong gave the visitors the lead in the 23rd minute. Town had been second best but took charge after a triple substitution with one of those changes Nathan Broadhead levelling on 68, before ex-Town loanee James Bree was dismissed for hauling down Davis on 85 and Sarmiento sent Portman Road into raptures seconds from the end.
In a season in which Town have broken so many records for a newly-promoted side, they continue to fight tooth and nail for promotion against the three recently-relegated, formerly established Premier League teams.