"We fully support Arden in his decision and look forward to welcoming him back home to the Tiger family at the appropriate time," LSU coach Ed Orgeron said in a statement.
Orgeron and Key said they would have no further comment on the matter.
Key, a junior, is considered one of the top returning pass-rushers in college football. He was second in the SEC this past season with 12 sacks and also finished with 56 tackles, 14.5 tackles for loss, 11 quarterback hurries and three forced fumbles.
He was named first-team All-SEC by the AP and second-team All-SEC by the coaches this past season.
Key had five sacks and led LSU with nine quarterback hurries as a freshman.