The 31-year-old former paceman for Worcestershire, Derbyshire, and Kent will be accessible from May 11 through July 31.
After the Black Caps’ five-game one-day international series against Pakistan in late April and early May, Henry is expected to report to Taunton.
Henry has primarily taken wickets in one-day international matches for his side.
In 65 games, he has taken 116 wickets, and in the last two ODI World Cup finals, against Australia in 2015 and England at Lord’s in 2019, he was on the losing team.
He has also participated in six T20 international matches, the latest of which was six years ago, and took 55 wickets in 18 Test matches.
Henry should be qualified for up to seven County Championship games as well as the whole T20 Blast, which takes place from May 20 to July 15, pending receipt of the required clearance and a visa.
Last year, Somerset avoided being threatened with relegation from the County Championship to finish seventh in Division One and go to the T20 Blast Finals Day, where they ultimately fell to eventual champions Hampshire.