The 26-year-old centre-back – who played every game in the Reds' promotion season from League One in 2018/19 – has joined the Bees for an undisclosed fee, believed to be around £3million.
He is the latest promotion hero to leave the club after his former centre-back partner Liam Lindsay move to Stoke City last week along with goalkeeper and captain Adam Davies.
Mads Andersen is now the only senior centre-back at Oakwell but the Reds are hoping to bring in at least one defender this week.
Co-chairman Paul Conway told the club's website: "He said: “We would like to thank Ethan for his dedication to the club for the last two seasons and wish him well. All proceeds from the sale will be reinvested in the squad in this transfer window.
"We offered Ethan a number of contract extensions but unfortunately, we were not able to convince him to stay. As I have stated before, the club will sell a player with less than one year left on their contract if they will not accept our contract extension offer.
"We believe we have mitigated this issue of shorter term contracts, as in the last 18 months and going forward most of the young players we have acquired have been signed to four or five-year deals. “he board has been very pleased with the new contracts signed this summer, with Cauley Woodrow, Cameron McGeehan, Jacob Brown and the permanent signing and new contract for Mike Bähre. We aim that there will be additional extensions to come from the squad and staff.
“We want to welcome again Luke Thomas, Brad Collins, Mads Anderson and Samuel Radlinger. There are other new exciting signings on the way. Finally, we want to thank all Reds fans for the continued strong support of the club, as seen in our record season ticket sales for the upcoming campaign.”