The 23-year-old, who has won four caps for the Netherlands, joins on a five-year deal.
"I'm really happy to be here," said Hoedt in a statement released by Southampton. "I think Southampton is a really big club, so it's a good next step for me to come here and to play in the Premier League and try to help the best I can."
Hoedt's arrival will spur further speculation about Southampton centre back Virgil van Dijk, who has submitted a transfer request. British media report that he is wanted by Liverpool and Chelsea.
Southampton, who also signed Poland's Jan Bednarek in the window, now have six senior centre backs. But Les Reed, Southampton's vice chairman (football), appeared to rule out an exit by saying he hoped Hoedt would develop "alongside his international team mate Virgil van Dijk" at the club.
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"Wesley is one of the most promising young defenders in Europe and represents a significant addition to our squad," he said.
Hoedt is Saints third summer signing, alongside five million pound Bednarek and 18 million pound midfielder Mario Lemina.
(Reporting by Neil Robinson; editing by Mark Heinrich)
