The deal also adds experience to Toronto's young roster.
In four years with the Quakes, Morrow has played in 71 regular-season games and two playoff games with 65 starts. Only Toronto midfielder Jeremy Hall (114) has played more MLS games.
"In Morrow, we are adding a young all-star calibre player to our back line," Toronto general manager Tim Bezbatchenko said in a release Tuesday. "Justin's steady presence, versatility and MLS experience make him a great addition to the club.”
The move beefs up the Toronto backline with the 26-year-old Morrow able to play both fullback and centre-back.
Morrow adds depth to captain Steven Caldwell and Doneil Henry in the middle of defence and will likely challenge Ashtone Morgan at left back.
Mark Bloom appears to have inherited the right back position from England's Richard Eckersley, who is likely to move on once the team buys out the $500,000-plus last year of a contract renegotiated to make the number more palatable in 2013.
Fullbacks Jonas Elmer and Ryan Richter and centre back Gale Agbossoumonde are also on the Toronto roster.
Toronto's defence, in a state of flux for the first half of the season, yielded 47 goals in 2013. That was down from 62 in 2012 but still ranked 14th in the 19-team league.
Morrow, who made US$139,562 last season with San Jose, has two career goals.
It's the latest in a series of off-season moves by Toronto FC as it bids to leave a 6-17-11 season far behind it.
Toronto unveiled Brazilian striker Gilberto last Friday, its newest designated player. Jermain Defoe is expected to join him in the January transfer window in a megabucks deal that will make the Spurs and England forward the face of the Toronto franchise.
While club officials have not confirmed the signing of Defoe, privately they see it as a done deal. The firing of Tottenham manager Andre Villas-Boas is a complication given the new boss might be more amenable to trying to keep Defoe.
TFC officials hope that any such wooing will be too little, too late.
Toronto also recently traded away the rights to goalie Stefan Frei and midfielder Bobby Convey for draft picks, acquired Brazilian midfielder Jackson from FC Dallas and re-signed goalie Joe Bendik to a new deal.
Toronto also appointed Greg Vanney as assistant GM and head of its academy.
San Jose picked Morrow in the second round — 28th overall — of the 2010 MLS SuperDraft.
Morrow also started for the U.S. in a 0-0 friendly against Canada last January and was on the bench for World Cup qualifying matches against Costa Rica and Mexico in March.
Judging from his Twitter account, Morrow is a Cleveland Browns fan with a sweet tooth and a taste for travel.
He played collegiate soccer at Notre Dame where he made 89 appearances in four seasons.
Having played under Notre Dame coach Bobby Clark while he was in charge of Stanford, Nelsen takes the Fighting Irish connection seriously.
Morrow joins forward Bright Dike as Notre Dame alumni on the Toronto team.