Gyokeres, 23, who spent the second half of last season on loan with the Sky Blues, has now signed on a more permanent basis, on a three-year deal.
They have also signed Ricardo Dinanga, 19, from Cork for their Under-23s.
Like Sheaf's move from Arsenal, the Sky Blues have again had to pay an undisclosed fee for Gyokeres.
"Viktor is a talented striker with good power, pace and movement, and great potential," said boss Mark Robins. "He showed a terrific attitude when he was with us on loan.
"He grew in confidence, and we hope that he will continue to show the same improvement and desire."
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