Switzerland international Fabian Frei is set to join Mainz from Basel, agreeing terms on a four-year deal, following fellow midfielder Johannes Geis' departure from the Coface Arena.
The Swiss champions announced the transfer on Tuesday, with Frei to undergo a medical before completing his move next month.
The 26-year-old - who scored against Liverpool at Anfield in the UEFA Champions League last December - has been with Basel for a decade and made 31 league appearances in their title-winning campaign last time out - the club's sixth in a row.
He missed two of those seasons after being sent on loan to St Gallen, while his international bow came in October 2011 against Wales.
Geis' switch to Schalke was confirmed earlier in the day, while Mainz have already made close-season moves for Leon Balogun, Florian Niederlechner, Yoshinori Muto and Danny Latza.
Coach Martin Schmidt, who took over from Kasper Hjulmand in February, will be seeking to improve on last season's finishing position of 11th.
