The 31-year-old arrives as a free agent having recently left league rivals Cheltenham Town and signs a deal until through until the end of the current campaign.
Alcock has 121 appearances to his name in green and white after signing a first professional contract at Huish Park in 2006.
Part of the Under-18s title-winning squad in 2005/06, the Truro-born defender made his first-team debut on the final day of the 2006/07 season having enjoyed loan spells at Weston-super-Mare and Tiverton Town.
Becoming a mainstay at right-back, the versatile defender became the club’s captain by 2010 amassing more than 100 outings.
Snapped up by Peterborough United in July 2011 he spent two seasons in the Championship with Posh before winning a Football League Trophy medal in 2014.
A two-season stay at Sheffield United followed prior to joining Doncaster Rovers, where he was part of the squad which achieved promotion out of League Two in 2017.
Returning to the West Country to spend the first half of the 2018/19 campaign with Cheltenham, Alcock now returns to where it all began.
Bolstering his defensive options, manager Darren Way told YTFC.net: “Craig knows the history and values of this football club which is what we need to move forward.
“I have trust in his determination to push the club forward and his experience and leadership will be valuable assets in part two of the season.”