Leicester-born Bell, who has worked in coaching for well over thirty years and was the first Englishman to complete his full coaching qualifications in Germany, previously worked with Siewert as TuS Koblenz, where he was head coach.
The 57-year-old had a successful seven-year association with Koblenz before becoming assistant head coach at Bundesliga side FC Cologne from 1996 to 1999.
After a spell as head coach at Dresden in 1999-00, Bell coached Mainz second team for four years and later returned to Koblenz where he served as academy boss and assistant head coach.
Bell has also enjoyed success in the women's game, leading Frankfurt to Women’s Champions League glory.
He joins the Terriers from his post as head coach of the Republic of Ireland’s women’s team, which he has held since 2017.
Siewert explained: "I am so happy to be able to welcome Colin to Huddersfield.
“He has a fantastic reputation in Germany as a top-quality coach, that he has really earned. He has been a head coach, a supporting coach and the head of academies at very high levels and can offer us so much.
"We first met when he was at TuS Koblenz and he was a big help to me early in my coaching career.
"We have kept in touch ever since and I am delighted he has accepted the offer to come and help us here at Huddersfield Town."