It had been rumored for a few weeks that Odejayi would make the move to Oakwell and now
The £200,000 fee breaks The Robins transfer record, set previously when The Reds splashed out £100,000 for Grant McCann in January.
There is also an undisclosed sell-on clause should Odejayi move from Oakwell in the future.
"Obviously its disappointing to lose Kay but the opportunity was too good for him to turn down and it's another record transfer fee."
" Kay has improved tremendously in the last 3 years and deserves the chance to test his ability in the Championship."
Odejayi will join up with ex Robins players Grant McCann and Martin Devaney.