In a statement on Wednesday morning, the Oakwell outfit revealed that the 26-year-old - whose contract ran out on Tuesday - has elected to turn down 'the most lucrative deal ever offered to a Reds player' to take on a new challenge.
The former Leeds United midfielder was a talismanic force in Barnsley's outstanding rise into the playoffs in 2020-21, with his feats securing him a place in the EFL Championship Team of the Season.
Mowatt and his representatives entered into contract talks with the Oakwell hierarchy last season, but an agreement was not able to be brokered with the Reds captain electing to move on under freedom of contract.
The Doncaster-born player was the subject of a failed seven-figure bid from Millwall in January and was also on the wanted list of several other Championship clubs, aware of his contractual position.
The likes of Nottingham Forest and Middlesbrough are among clubs to have also expressed interest in Mowatt, but West Brom are now poised to confirm his signing, with the chance of linking up again with Ismael likely to prove pivotal in Mowatt's decision to head to the West Midlands.
He will undergo a medical with Albion today.
A Reds statement said: "Barnsley Football Club can confirm that captain Alex Mowatt has now left Oakwell upon the conclusion of his contract.
"A key player in each of the last three seasons, a regular under three separate head coaches, Alex’s efforts were exemplary and the club entered discussions over an extension to his contract at the earliest juncture.
"Despite the most lucrative deal ever offered to a Reds player, Mowatt has decided against extending his Oakwell career.
"The club would like to thank Alex Mowatt for his dedication and professional attitude, particularly as captain over the last two seasons and wishes him well in his future endeavours."
Posting a message on his Twitter account, Mowatt said: "To all the Barnsley fans. Thank you for the support you have given me over the last four and a half years.
"It has been an honour to captain this incredible club and share some unbelievable moments that will live with me forever.
"We came to achieving something incredible last season and although the lads couldn't quite get across the line, it was an amazing journey and fantastic to see how far we had come since making the great escape.
"However, it is time now for me to take on a new challenge, which means this is goodbye.
"Again, thank you so much for your tremendous support. This club, and you, the fans, will always have a special place in my heart and I wish everyone at Barnsley the best for the future."
Mowatt joined the Reds from Leeds in January, 2017, effectively as the replacement for Conor Hourihane, who headed to Aston Villa in that January transfer window.
The midfielder endured a difficult start to his Oakwell career and was sent off on his home debut against Wolves.
He spent the 2017-18 season on loan at Oxford United and his time at Oakwell looked to be at a crossroads before being galvanised by the arrival of Daniel Stendel, with Mowatt playing an instrumental role in the Reds' League One promotion campaign of 2018-19, which saw him earn a place in the PFA Team of the Year.
Mowatt continued that outstanding form back in the second tier, which saw him take on the captaincy.
The midfielder has been named as the club's players' player of the season for last three campaigns.