In doing so, Uwe Rosler has ticked another box in his plans for the new season.
Turnbull arrives at Elland Road principally as cover for first-choice keeper Marco Silvestri.
United have signed Turnbull on a two-year contract after the 30-year-old completed his recovery from a back injury.
Turnbull missed Barnsley’s run-in last season after undergoing surgery on the problem but Leeds monitored him throughout the summer and finalised a free transfer this afternoon.
He is expected to play in one of United’s friendlies at York City and Tadcaster Albion this evening.
The former Middlesbrough and Chelsea player had 12 months remaining on his deal at Oakwell but Barnsley will use Adam Davies as first choice during the forthcoming League One campaign and were looking to remove Turnbull from their wage bill.
The transfer takes United’s tally of summer signings to six, following on from the arrival of midfielder Tom Adeyemi from Cardiff City yesterday and leaving a winger as Rosler’s remaining priority.
Turnbull spent the bulk of his career with Middlesbrough - a period in which he moved out on loan repeatedly - before Chelsea unexpectedly signed him in 2009.
He was rarely used at Stamford Bridge but his spell in London coincided with Chelsea winning the Champions League. A move to Doncaster followed before Barnsley took him on last summer.
Leeds were effectively without cover for Silvestri after releasing Stuart Taylor at the end of the 2014-15 season.
The club landed Charlie Horton from Cardiff last month but the 20-year-old is being lined up to play for their development squad next term.
Rosler admitted yesterday that his work in the transfer market was “95 per cent complete” after Cardiff agreed to send Adeyemi to Elland Road on a season-long loan.
Leeds want to add an out-and-out winger to their squad and the club are understood to have enquired about Floyd Ayite, a Togo international who plays for French top-flight side Bastia.
Ayite, 26, is an ex-Bordeaux and Reims player who finished last season with six goals as Bastia fought their way clear of relegation.
He is likely to cost a seven-figure fee, however, and United have identified other options including Catania’s Brazilian winger Raphael Martinho.
Martinho was targeted by owner Massimo Cellino last season but issues with the 27-year-old’s passport are thought to have complicated discussions.