Isaacson's Heineken Cup experience includes playing in a quarter-final with Newcastle Falcons against Stade Francais Paris at Parc des Princes in last season's tournament.
Commenting on the signing Director of Rugby Phil Davies said, "James is a versatile player who can play equally as well at hooker and prop. He will provide us with plenty of options in the forwards, in what will probably be one of the toughest seasons to date. In front row terms, James is a young player who will hopefully develop with us and I look forward to seeing him fulfil his potential in the years to come."
Leeds Tykes will face Calvisano, Cardiff Blues and Perpignan in Pool 2 in next season's Heineken Cup.