Chaplin, who the Oakwell club tried to sign last summer before he left Portsmouth for the Sky Blues, has arrived for an undisclosed fee.
It is understood that the fee could eventually be worth over £1million with potential add-ons.
Barnsley reportedly saw a £700,000 offer rejected for Worthing-born Chaplin by Coventry recently, but the Reds maintained their interest in the 22-year-old, who moved to the Midlands in a £500,000 deal in August 2018.
On finally joining the Reds, Chaplin, the club's tenth close-season recruit, said: "I am happy to get it over the line after so much speculation.
"I can now go out and meet my new teammates and the gaffer and hopefully show the fans what I am all about."
Commenting on the first incoming deal of his tenure at Oakwell, new chief executive officer Dane Murphy 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 and so we are delighted to get this one done.
"We believe the Reds fans will be excited by what Conor can offer us."
Chaplin scored eight goals in 33 appearances for Coventry last term and re-unites with Luke Thomas, who spent a season-long loan in the Midlands in 2018-19 before moving to Barnsley earlier this summer.