Two goals from Jason Euell and one from substitute Matthew Paterson secured an impressive victory for Southampton that severely dents Ipswich's promotion bid.
Euell headed home an Andrew Surman free-kick for his first before capitalising on a Pim Balkestein mistake to slot home his second.
Paterson drove in from the edge of the box to complete the rout.
Saints also had headers from Jan-Paul Saeijs and Chris Perry disallowed by referee Andy D'Urso for fouls.
Ipswich boss Jim Magilton told BBC Radio Suffolk: "It's certainly the worst since I became manager, without a shadow of a doubt. It was three (nil) going on six.
"It was not acceptable. The crowd had every right to voice their opinion.
"You have to have a few words after that performance. You want to know where it came from. It stunned the life out of me. I didn't see it coming."
Southampton boss Mark Wotte: "Jason Euell has done fantastically for us tonight. I am pleased with him.
"When I started this job I gave him a substitute role three times, and every time he had a good impact - so I put him in the first XI.
"His contract is expiring, so he has personal reasons to play well. He feels confident, like most of the players, and he has done brilliantly."
Ipswich: Richard Wright, Bruce, McAuley, Balkestein, David Wright, Norris, Miller, Quinn (Garvan 61), Civelli (Walters 46), Counago, Stead (Lisbie 46). Subs Not Used: Supple, Campo.
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