The 27-year-old Scotland international joined from Peterborough in February and was available on a free transfer after his contract expired at the end of the season.
He has agreed a two-year contract in East Yorkshire, the club confirmed in a statement on Twitter.
Boyd made a big impact on his arrival at the KC Stadium, scoring four goals in five games in March to win the Championship player of the month award. He did not score again as Hull stumbled slightly before sealing second place on the final day of the season, but ended the campaign with 11 goals – seven of which came for Peterborough.
Prior to arriving at Hull, Boyd was expected to sign for Nottingham Forest in the January transfer window but the move fell through due to an “inconclusive eye test” – much to the frustration of then Forest boss Alex McLeish.
Hull manager Steve Bruce is understood to have a transfer budget of up to £15million, with a goalkeeper a priority.
Another loanee, Fulham’s David Stockdale, ended the season as first choice but it is believed Bruce does not want to pay the £3million valuation.