Meredith, 27, will also be available to play in the Metro Bank One-Day Cup group stages, plus a “managed number” of Vitality County Championship matches.
“I’m looking forward to the challenge of playing in English conditions and I can’t wait to get started,” Meredith said. “Somerset are the current Vitality Blast champions and I’ll be doing everything I can to help them retain the title.”
To date, Meredith has played five T20Is for Australia, taking eight wickets at 23.50, plus a one-off ODI against West Indies, all in 2021.
Across his whole T20 career, Meredith has claimed 114 wickets in 91 matches, at an average of 25.42 with a best of 4 for 21 for Hobart Hurricanes in the Big Bash League. He has also represented Mumbai Indians and Punjab Kings in the IPL.
“Somerset are one of the biggest names in the English domestic game and I’m excited to be able to play a part in their journey this year,” he added.
Somerset Director of Cricket, Andy Hurry said: “We are extremely pleased to have been able to sign a cricketer of Riley’s ability and experience. He bowls with real pace and skill and will add further experience and a different dimension to our bowling attack.
“He is driven to achieve further international and high-profile global franchise recognition, and we look forward to him playing a key role for us this summer.”