The 31-year-old back-rower, who played for Samoa in the 2014 Four Nations Series, has agreed a two-year deal with the Black and Whites, who also have an option on a third season.
Hull coach Lee Radford said: “Frank is a quality signing. He is going to be a real hit in this competition and, as a former club captain at the Bulldogs, a Grand Final winner and a top New Zealand international for some years, his CV speaks for itself.
“The fact we had to fight so hard against three or four other clubs in Super League to get his signature and Canterbury’s reluctance to see him go speaks volumes and is a real sign of his quality and the stature of the man.
“Having spoken to him he is desperate to come over here, make an impression and be successful at this club, which is what we want.”
Pritchard played 144 times for Penrith in eight seasons from 2003-10 before switching to the Bulldogs, where his 92 appearances to date include two Grand Finals.
“The English Super League is continuing to grow as a competition and to be able to go over there and continue as a professional athlete is exciting,” he said
“Hull were the best fit for me. Having spoken to the guys there I can see what they’re trying to build and what they want to achieve and it will be exciting to be part of that.”
Bulldogs coach Des Hasler, who had a brief spell with Hull as a player, said: “Frank has been a leader amongst the playing group since arriving at the club and I know he’ll want to continue to perform on the field through until the end of the season.”