After New Zealand’s scheduled World Test Championship final against India in late June, De Grandhomme, 34, is expected to join the county.
He previously competed in the competition for Birmingham Bears in 2017 and 2018, helping them reach the final in 2018.
Due to an ankle injury, De Grandhomme has been out of international play for the past year.
“I’ve really loved playing county cricket, and I’m very excited to represent Hampshire,” De Grandhomme said.
“I’m hoping to help with some positive results to a very talented group of players.”
De Grandhomme could play alongside Australia limited-overs specialist D’Arcy Short, Hampshire’s other overseas signing for the Blast.
With the World Test Championship final at Ageas Bowl ending on June 22, he may be available for Hampshire’s final eight group matches.