Hofmann made 22 top-flight appearances from the bench for Dortmund last season, in addition to four starts, but will expect more regular action at the Coface Arena.
And the Germany Under-21 international is confident he can make an impact at his new club.
"I am looking forward to my time in Mainz and the prospect of being able to make a contribution in the Bundesliga," he said. "I'm sure we will have a successful time together."
Mainz general manager Christian Heidel expressed delight at being able to bring in a player of Hofmann's calibre.
"We have been in contact with Jonas Hofmann for a long time and are very pleased that he will now strengthen us this year," he added. "Jonas is flexible in midfield and is particularly quick down the wings."
Mainz will also have the services of defender Philipp Wollscheid this season after Bayer Leverkusen confirmed they would be loaning the player out.
Wollscheid has made 51 league appearances for Leverkusen.