Ronchi will play five matches for Somerset as a replacement for fellow Kiwi Corey Anderson who was injured in the recent Test series against England.
Born in New Zealand, but raised mostly in Australia, Ronchi became the first man since Kepler Wessels 20 years ago to represent two full ICC member nations at international level.
He is currently representing New Zealand in the one-day international series against England and will be well-acclimatised to English wickets and conditions by the time he arrives in Taunton.
In 2006-07 he made the fastest century in Australian domestic one-day history as he reached three figures from just 56 balls against New South Wales. He also scored a 51 ball century in the 2007-08 Pura Cup against Queensland, with the second 50 coming from just 11 deliveries.
The 34-year-old has scored 263 runs in 15 T20 Internationals at an average of 32.87, while he averages 29.45 in ODI’s with a highest score of 170*.
Ronchi made his Test debut against England at Headingley in May this year, scoring 88 in the first innings.
His highest score in the current one-day series with England was the 33 he scored at the Oval last weekend, although his scoring opportunities have been limited as he has mainly come to the crease in the final overs of the New Zealand innings when the top order have already accumulated large totals.
Somerset Director of Cricket Matt Maynard said: “Luke is a quality player with plenty of international experience. Any new addition must add something to the squad and he will certainly do that. He is an excellent ‘keeper, an explosive batsman and I am sure that our members and supporters will enjoy the brand of cricket that he plays.”
Ronchi will join Somerset ahead of the NatWest T20 Blast match at Middlesex on June 26. He will also be available for selection for the home fixtures against Glamorgan (28 June) and Gloucestershire (3 July) and the away fixtures at Kent (10 July) and Surrey (17 July).
He will not be available for Somerset’s final two T20 Blast games against Hampshire at the Ageas Bowl on 23 July and at Taunton on 24 July.
Meanwhile Maynard has confirmed to The Bristol Post that Marcus Trescothick is “taking a breather” from T20s and will not keep wicket in the derby against Gloucestershire tomorrow. Temporary signing Michael Bates, who made his first-team debut for Somerset in this week’s LV=Championship victory over Nottinghamshire, will keep instead.