The Tykes headed into the game on the back of a morale-boosting midweek victory over Burton, and Tom Bradshaw struck early on after good work from Barnes to pick up where they had left off.
The visitors rarely posed a serious threat and Zeki Fryers condemned them to their second 2-0 defeat in four days, after a loss to Brentford on Wednesday, with a well-taken free-kick.
Barnsley's convincing win moves them up to 16th place in the Sky Bet Championship. The outcome leaves Birmingham still without an away win since the final game of last season, without a goal in their last four games, and sitting in the relegation zone.
Barnsley boss Paul Heckingbottom was pleased with the way his players neutralised Birmingham's attacking threats.
He said: "We've probably dominated games as much, if not more, than that at Oakwell and either drawn or lost, so it's nice to put in a good performance and get a good win.
"We were in a strong position early doors, which was good. We always want good starts, of course we do. I think the most pleasing bit for me was how we dealt with them."
The Blues started brightly when former Barnsley captain Marc Roberts glanced his header into Adam Davies' hands from a David Cotterill free-kick inside the first two minutes.
From that free-kick, the goalkeeper launched a counter-attack and set the in-form Barnes away.
The Leicester loanee delivered a low cross from the left and Barnsley's top scorer Bradshaw slotted into the roof of the net to put the home side in front.
Joe Williams attempted another wonder goal, after his goal of the season contender in midweek, but this time he could only steer his drive at Birmingham goalkeeper Tomasz Kuszczak.
The home side went close to doubling the lead twice after captain Angus McDonald headed Barnes' deep cross just wide and Jonathan Grounds' well-timed block denied Lloyd Isgrove his second goal of the week.
Birmingham created a rare chance in the second half as Cotterill broke and played Jota into the box. The Spaniard could not manage to find the target though, as summer signing Liam Lindsay made a vital sliding interception.
Barnsley doubled their lead in the 68th minute as Fryers scored his first goal for the club since arriving in the summer from Crystal Palace. The left-back hit a low free-kick from the edge of the penalty area that found the far bottom corner.
George Moncur saw two similar bending efforts drift just wide, before Jota sent one ambitious shot straight into Davies' arms and then fired another well off target.
Kuszczak managed to keep the scoreline respectable, with a tremendous double save at the death. Brad Potts broke free and squared it to a free Bradshaw but Kuszczak rushed out to deny the Welshman a second, then jumped up to pounce on Andy Yiadom's drive from the rebound.
Heckingbottom added: "The long ball in terms of hitting (Sam) Gallagher or (Lukas) Jutkiewicz, we generally competed or picked up the second, but we still tried to play. It was really important we did that.
"I've just been speaking about hitting the target.
"They think I'm silly when I shout it that much, but cheap goals, you get them.
"I'll not stop shouting at them if they don't hit the target. It's not trying to score, but trying to hit the target. For Fryers' free-kick, he's aiming for goal or somebody to get a touch, that's his idea."