The 49ers said in a press release that they received an undisclosed draft choice in the trade for Harold. Per NFL Network's Mike Garafolo, the 49ers will receive the Lions' seventh-round pick in the 2020 NFL Draft.
Harold recorded 34 tackles and two sacks in 2017 while playing in all 16 games for the third straight season. The 24-year-old has collected 85 tackles, five sacks, two fumble recoveries and one forced fumble in 48 career games since being selected by San Francisco in the third round of the 2015 NFL Draft.
The 6-foot-3, 257-pound Harold reportedly was on the roster bubble with the 49ers.
"Our entire organization would like to thank Eli for his hard work and contributions to our team and the Bay Area community over the last three years," 49ers general manager John Lynch said. "He always represented the 49ers with class and has been a tremendous teammate. We wish Eli and his wife, Kelsey, nothing but the best in the years to come."
Harold was also a player who kneeled alongside former 49ers safety Eric Reid last season during the national anthem to protest racial injustice. The linebacker, however, has chosen to stand during the anthem this preseason.
"I understand why everyone wants to ask the questions, but I just don't want to get into that right now," Harold said of his new anthem stance, per the San Francisco Chronicle. "I'm trying to focus on ball. Me and my wife had the discussion, and we feel it is best right now that I don't answer those questions and try to stick with ball, if you don't mind."
In Detroit, Harold will join fellow linebackers Devon Kennard, Jalen Reeves-Maybin and Jarrad Davis.