Albion chairman Tony Bloom says Sami Hyypia quit as manager "for the benefit of the team and the club".
The club confirmed Hyypia's resignation in a statement.
Bloom, who recently left the country for his annual winter migration in Australia, said: "Sami offered his resignation and we reluctantly accepted earlier today.
"Sami has worked extremely hard since joining the club last summer. Despite some good performances, including most recently at Wolves, results haven't gone as well as we all had hoped and expected.
"Sami is an incredibly honourable man. He remains a hugely respected and very popular figure at our club. He has decided to step down for the benefit of the team and the club.
"We are all very disappointed, but we respect Sami's decision and we all wish him well for the future."
Bloom is now likely to fly back from Australia to oversee the appointment of Hyypia's successor.
Assistant Nathan Jones will take charge for the Boxing Day clash against Reading at the Amex.
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