The 25-year-old spent the first half of the season on loan at League One Rotherham and made a name for himself, scoring 13 goals in 24 appearances.
That attracted the attention of a number of clubs, including the Millers, but Barnsley have won the race, pinning him down to a three-and-a-half-year deal.
Boss Paul Heckingbottom told the club’s official website: “Kieffer is an excellent addition to the side and we’re really pleased to bring him here.
“It’s a deal that the club has worked hard on and got done quickly, so I’m delighted with that.
“He’s a player in top form and will bring something different to improve the side. We were after a player of his qualities, who can offer something different and improve the starting eleven.
“His height will trouble defenders and cause problems, but as a player he’s got much more in his locker and will score different types of goals.
“Kieffer has already been in the local area and knows South Yorkshire now, so I expect him to settle quickly and work hard to get into our team.”
Paul Huntington says Preston's play-off hopes this season hinge on them getting the balance right on and off the pitch.