A 23-year-old forward capable of operating across the front line, he joins from Toulouse for an undisclosed fee.
Barnsley co-chairman Paul Conway called Iseka: "a unique and aggressive attacker with extensive experience in top-flight football across the continent.
"Aaron’s quality and understanding of the game will help our squad to grow, and we are confident that he will also make his own impact on the field."
The Belgian under-21 international spent last season on loan at Metz, having made his breakthrough at Anderlecht. He has also had loans at Marseilles and Zulte Waregem, where he scored a winning goal against Lazio in the Europa League three years ago.
Iseka, brother of Chelsea's Michy Batshuayi, has signed a four-year contract at Oakwell.
The Reds have also signed Devante Cole and Josh Benson this summer. In this understandably frugal transfer window, few fees have been paid, but the compensation Barnsley received after Valerien Ismael, Paul Murray, sport science coach Jonny Northeast and video analyst Jack Riley joined West Bromwich Albion has allowed them to buy the two Belgians, plus midfielder Benson from Burnley.
The Reds are at Cardiff City on the Championship's opening weekend.