The suspension was first reported by the New York Daily News, which said Marshall tested positive for Adderall.
"I really just want to apologize for this mistake," Marshall tweeted on Tuesday. "I inadvertently took medication without an exemption from the league on one occasion. It has not happened since & will not happen again. Already looking forward to 2017 and doing whatever I can to help this team win."
Marshall made the Jets' roster as an undrafted free agent last season out of Ohio State. He finished with 14 receptions for 162 yards and two touchdowns, missing six games because of a torn labrum in his shoulder. He also returned punts and kickoffs.
Decker is their most accomplished receiver, but he's recovering from shoulder and hip surgeries and could become a salary-cap casualty in the coming months. General manager Mike Maccagnan stopped short last week of committing to Decker for the 2017 season, but he said they'd wait until he's healthy before making a decision.