The 24-year-old off-spinner made his first-class debut in 2014 while a student at Cardiff University.
They also have Yorkshire all-rounder Will Rhodes to bed in alongside their two 2017 mid-season signings Sibley, Somerset batsman Adam Hose, as well as a fully-fit Olly Stone.
Following Ian Bell#8217;s removal from the captaincy in August, Jonathan Trott became Championship skipper, while New Zealander Grant Elliott took over the Birmingham Bears captaincy #8211; and is expected to do that job again in 2018.
Thomson played in Warwickshire#8217;s final two County Championship games of last season, following confirmation of the Bears#8217; relegation to Division Two, scoring 70 runs in four innings.
Warwickshire#8217;s slow bowling department will now have four main choices next season, led by senior professional Jeetan Patel, the 37-year-old New Zealand Test off-spinner, who is in the final year of his Bears contract, having now retired from international cricket.