South Africa's youngest ever Test captain will link up with compatriot Albie Morkel at Taunton, having recommended the 23-year-old all-rounder as the county's second overseas player.
Smith showed his liking for English conditions in 2003, when he struck two double centuries at the start of the 2-2 drawn series with England.
The left-handed opener was pinpointed by Somerset after they failed in a bid to sign New Zealand's Stephen Fleming.
Although yet to be given formal permission to take up the role by his UCB bosses, Smith, also 23, will be available for almost the entire season.
South Africa's itinerary in West Indies finishes on May 15 and there is then a five-month gap to their next fixtures.
Morkel, a hard-hitting left-hander who bowls right-arm fast-medium, broke into the South Africa one-day team earlier this year and his availability in the opening weeks of the 2005 summer will depend on whether he merits selection for the Caribbean tour.
In Smith's absence, England batsman Marcus Trescothick would be an option to lead the side while Australia captain Ricky Ponting, a success in his brief stay last summer, might yet be engaged for another short stint, international commitments permitting.