Smith joins on a free transfer from the Pilgrims, penning a two-year contract at Huish Park.
The 24-year-old made 31 appearances for Derek Adams’ side last season, helping them secure automatic promotion to Sky Bet League One.
He becomes the Glovers’ second summer addition after Rhys Browne joined from Grimsby Town for an undisclosed fee.
Smith, who has four caps for Republic of Ireland Under-21s, was part of Watford’s youth setup having first come to prominence in 2010 having starred in the television show Football’s Next Star.
Having signed a professional contract with the Hornets, the Irishman had brief stints with Gillingham and Stevenage, before a move to AFC Wimbledon.
He featured in the Dons’ starting line-up that won the 2015/16 League Two play-off final at Wembley, before opting to join Plymouth where he earned his second consecutive promotion last term.
“Having come up against Connor in the last two seasons, we’ve been very aware of his history,” boss Darren Way told YTFC.net.
“I feel very fortunate to capture the signing of a player off the back of two promotions in as many seasons and it’s a massive coup for this football club.”
Are you interested in sponsoring Connor for the 2017/18 campaign? Please contact Dave Linney on 01935 847872 or email firstname.lastname@example.org for more information.