Day, 25, is an Indianapolis native who was runner-up for the 2011 Indiana Mr. Football award.
He entered the league as a fourth-round choice of the Jaguars in 2016.
Day has played 56 career games, with two starts, with the Jaguars (2016-17) and 49ers (2017-19). He has 53 tackles, 14 tackles for loss, six sacks and four passes defensed.
Day was a key member of the 49ers’ postseason run in 2019. He started all three of San Francisco’s playoff games last season, making three tackles, including one for loss.