The white-rose rivals’ first clash since March 2014 comes with Town third and unbeaten in the league and their visitors seventh after successive home wins.
Wagner, whose side won 2-1 at Newcastle United before Tuesday’s 1-1 draw at Aston Villa, watched Barnsley’s 3-2 victory over QPR on Wednesday and was impressed.
“I liked their togetherness, their fighting attitude and their ability to play different styles,” he said.
“They pose danger at set-pieces and they deserved their win against QPR.
“I think it was a good time for us to play two relegated sides in Newcastle and Aston Villa.
“But with those promoted like Barnsley it’s different because there is often momentum.
“I think they will be the most difficult team we have faced so far - because they play as a team.”
Wagner brushed off the theory Town might find it hard to refocus after playing in front of bumper crowds at St James’ Park and Villa Park.
“We’re looking forward to being back at home and having a local derby in front of a lot of fans,” he added.
“I’m happy a neighbour like Barnsley came up and are in our division, and this is a big match for both of us.
“I trust in my players and they know what is needed, to be brave, have self-belief and be competitive.
“We as coaches will try to make the details right, then we will have every chance to be sucessful.”
Town have a major doubt over forward Joe Lolley, who took a knock against Villa.
But otherwise it’s a cleanm bill of health, with midfielder Philip Billing (ankle) and forward Harry Bunn (hamstring) back in training.