The 32-year-old is currently playing for New Zealand in the Champions Trophy and last weekend was named man of the match in the win over Sri Lanka in Cardiff.
"I'm looking forward to joining up with the Glamorgan squad at the end of this month," he said.
"I've spent a bit of time in Cardiff over the past few weeks and have enjoyed playing at the SWALEC Stadium. Spin bowling plays an important role in T20 cricket."
McCullum is the older brother of current New Zealand captain Brendon, who also had a spell with Glamorgan in 2006 and scored the county's fastest half-century in T20 cricket from just 18 balls against Worcestershire.
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