Hamburger have signed Toni Leistner from Queen’s Park Rangers, whilst Hannover 96 sign experienced Swedish left-back Niklas Hult.
Holtmann leaves Mainz after four years, having previously played for Braunschweig, having made just 29 appearances for the Bundesliga club.
The 32-year-old has signed a one-year contract with the Red Shorts, who are looking to get back into the Bundesliga at the third attempt.
They have signed defender Abdourahmane Barry after he impressed in a trial with the club.
Tuesday was a busy day in the 2. Bundesliga with six teams bringing in new players as the countdown to the new season continues.
Eintracht Braunschweig and VfL Bochum have both announced returning players over the past few days, with VfL Osnabrück making a double swoop as well.
He moves to the Millerntor after a breakthrough season with the Zebras, contributing 11 goals and 5 assists although the side eventually fell away from a promotion place.
They have signed Japanese defender Sei Muroya, cancelled the contract of former captain Marvin Bakalorz and extended with Marlon Sündermann.
The 31-year-old has signed from relegated side SV Wehen Wiesbaden.
Despite only rejoining the club last summer and having a contract until 2023, Zieler was unceremoniously told he would play no further part in Kenan Kocak’s plans in the upcoming season.