"My heart is heavy but I'm grateful that I got to know her," he tweeted Wednesday. "I'll cherish the memories, conversations and her consistent support. Rest well."
The actors co-starred in 2015's Star Wars: The Force Awakens.
Boyega, a lifelong Star Wars fan, often asked Fisher for advice in between takes. "She is hilarious," he told Yahoo!'s Katie Couric last year. "One thing she did say to me was, 'You're going to have stalkers—guaranteed.' I'm doing well so far. I'm not sure that's going to last."