The 28-year-old right-hander, who has also represented Australia U19s and South Australia, averages 48.80 from his 23 one-day internationals with Holland and has a first-class average of almost 40 from 46 games.
Somerset had been looking to bring in batting reinforcements after losing Nick Compton, Alviro Petersen and Chris Jones from last season’s squad. And doubts remain about Craig Kieswetter’s availability for the coming campaign due to his eye injury.
Somerset director of cricket Matt Maynard told the club’s website: “It is really good news that Tom has joined us for this season.
???He is a highly-regarded batsman with a really good track record in all forms of the game and will be a great addition to our squad.”
Cooper said: ???I am really very pleased to have signed to play cricket for Somerset in 2015.
???I have always wanted to play county cricket in England and have heard a lot about Somerset so I am looking forward to getting going in April.???
Cooper will not count as an overseas player. Pakistan left-arm spinner Abdur Rehman will be Somerset’s overseas star in 2015.