The 22-year-old forward, who becomes head coach Daniel Stendel’s tenth signing of the summer, joined from League One side Coventry City for an undisclosed fee.
With Portsmouth throughout his formative years, Chaplin signed professional terms with Pompey in 2014 after scoring 11 goals in 11 games for their under-18 side. He went on to make his senior debut for the club later that year and in April of 2015 he scored his first league goal against Morecambe.
At the end of that season he was named League Two's 'Apprentice of the Year', such was his impact at a tender age.
The following season was his first full campaign for Portsmouth and he bagged 11 goals in 38 appearances, often as a substitute. He'd score a further 13 times before last summer signing with Coventry City. With the Sky Blues, last term he found the net eight times in 31 appearances.
A diminutive forward able to operate in a number of positions who possesses real pace and is a scorer of spectacular goals, including free-kicks, Chaplin was thrilled to put pen-to-paper at Oakwell.
"I'm so happy to get it over the line,” he said. “There's been a bit of speculation over the summer and I'm glad to be here, ready to get started.
"I'm looking forward to meeting up with the lads in Germany and getting to know everyone ahead of an exciting season in the Championship."
Dane Murphy, the Reds’ chief executive, added: “Conor Chaplin’s profile is a rarity in English football and the club believes he will fit seamlessly into our playing philosophy. He has been coveted by our staff for some time.
"This latest addition further improves our young, dynamic attacking core. Most importantly, he is a person of high character and in our research we’ve found that he is well-liked in each team he has been a part of.
"We are delighted to have him oand think he will excite the Reds’ fans."