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 upon his arrival at the KC Stadium, scoring four goals in five games in March to win the npower 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 Posh.
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 boss Steve Bruce is understood to have a transfer budget of up to ?15million, with a goalkeeper the next priority.
Another loanee, Fulham's David Stockdale, ended the season as the club's number one but Bruce is believed to be unwilling to meet the Cottagers' £3million valuation.