The Dutchman was a key part in Southampton's promotion-winning campaign from the second tier in 2011-12, and made 25 Premier League appearances in the following season.
However, Hooiveld has fallen down the pecking in order in the past two years, and had a loan spell at Norwich City earlier this season.
It was announced on Monday that the defender has joined Millwall, who are toiling in the Championship's bottom three.
"I want as much success as possible - as many wins and as many clean sheets as we can - which is just what every footballer hopes for," he told Millwall's official website.
"It is very important [to be playing regularly] and I'm no different from anyone else.
"I had a very nice and positive conversation with the manager [Ian Holloway]. We had a good connection.
"Every game from now on is massive. It isn't just one game that decides it - there are 20."
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