Over 11 starts this season, Day recorded 40 tackles, 7.5 tackles for loss, nine quarterback hurries and one sack.
The 6-foot-2, 285-pound junior started eight games as a sophomore last season and played in 13 games as a reserve in 2012.
"After talking with my family, friends and coaches at Notre Dame, I've decided to return for my senior year with the Irish," Day said in a news release. "While the process leading up to this decision was difficult, ultimately my decision to return was easy. I love this school, my teammates and this coaching staff. I just felt it was in my best interest to play another year for Notre Dame.
"I believe we've got an opportunity to have a special season in 2015 and I wanted to be a part of that success. I was blessed to play in the national championship game as a freshman in 2012, and I want to do everything in my power to reach that stage again with my guys."
Notre Dame finished 8-5 this season after beating LSU in the Music City Bowl on Dec. 30.