Middlesex begin their T20 Blast campaign against Kent at Lord’s on Friday, and will host Surrey next on Sunday.
“Being able to call the Home of Cricket my home ground for any period of time is something I never thought could have been a possibility, and I am equally excited to be able to play in a competition with the quality of the Vitality Blast”, said Boyle.
Fulton added: “I know Matt well from my time in New Zealand, coaching him at Canterbury for the past four seasons.
“He is a tall left-hander with a career T20 strike rate of over 150 and will add power to our top order.
“I have no doubt we will see Matt playing international cricket for New Zealand in the future, and I am looking forward to seeing him make a name for himself at Middlesex.”