"I have made the decision to retire from football," Penisini wrote. "I'm definitely going to miss my teammates and the coaching staff but I'm glad I got to experience it. I'm happy and excited for whatever life has for me."
Penisini, 25, started in 12 of his 32 career games, registering 49 tackles, four tackles for loss, a sack and a fumble recovery. Detroit selected him in the sixth round of the 2020 NFL draft out of Utah.
"For my family, friends, teammates, coaches and all the people who supported my dream along the way I appreciate and love you guys," Penisini wrote. "Go Lions #OnePride."