Ishaku polled aggregate votes of 369,318 to defeat APC candidate Aisha Jummai Alhassan who polled 275,984 after the Saturday's supplementary election.
State Returning Officer for Taraba, Prof Mohammed Kyari declared Ishaku winner at the INEC Collation Centre, Jalingo late morning on Sunday.
But APC Agent, Gambo Umar said his party will challenge the result at the tribunal, alleging irregularities in some areas during the election.
PDP Agent, Rebo Usman, however hailed the outcome of the election and said it was victory for the people of the state.
He said his party is expecting congratulatory messages from the APC.