Wednesday manager Alan Irvine told the club website: "He is a young, athletic defender who I have been aware of for some time.
"It is no secret that we have been looking to strengthen our defence and Reda is a quality addition."
He added: "He is composed on the ball, very strong in the air and in the tackle and has plenty of experience for a player of 22.
i think you've got a good buy who could learn a lot from an experienced defender
plymouth247
"I would like to thank [chairman] Milan Mandaric for cutting short his holiday to help me bring in Reda to Hillsborough and I am sure the reward will be some excellent performances on the pitch."
Johnson, a Benin international, started his career in his native France, playing for Gueugnon and Amiens before his move to Plymouth.
Source: BBC Sport
Source: BBC Sport