Liebherr added: "Alan has a strong track record and impressed us with his vision, commitment and ambition. We look forward to working with him."
Pardew took West Ham to the FA Cup final in 2006, but his spell at Charlton was less successful with the club languishing at the foot of the Championship table when he departed.
But the former Reading boss said he was relishing his latest challenge.
"Everything is here in terms of quality of stadium, training facilities and, now, financial stability," he said.
"I'm look forward to working with the squad and with the executive management as part of a team committed to achieving progress.
"Of course, I will bring my own style of football management to the club, but as part of a plan being evolved it to achieve long term success."
Stewart Henderson has been in temporary charge of Southampton and is taking the squad through pre-season training and preparing them for a friendly against Ajax on Saturday.
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