The 23-year-old, a one-cap Scottish international who arrived at Town from Manchester City in 2023, heads to Oakwell after impressing out on loan at Barnet in League Two last season, where he played 44 games.
Slicker said: “After last year, the next step was key for me, and I think this is the perfect step.
"It’s a massive club with a rich history and that’s what I want. I want to play in front of great fans in a stadium like this.”
Cieran Slicker impressed out on loan at Barnet last season (Image: Ross Halls)
Barnsley head coach Daniel Stendel said: "We're very happy to bring Cieran to Barnsley. He is a goalkeeper with excellent quality, good experience for his age and a strong mentality.
"He has worked in top environments and shown his ability during his loan spell last season.
“Standing at 6ft 3in, he is a commanding presence in the penalty area and a modern goalkeeper who is very comfortable with the ball at his feet.
“We believe he will fit our style of play very well, be an important player for us, and we are excited to work with him."
Slicker's departure leaves Christian Walton, Alex Palmer, David Button and Henry Gray as Town's senior goalkeeping unit. Button is out of contract this summer, but the club said he was in talks to remain in Suffolk when they released their retained list.