Cain Velasquez might be behind bars, but he's still finding a way to let his daughter know he's thinking about her on her birthday ... penning a sweet message to the 13-year-old from jail.
The UFC legend posted the note to his Twitter page on Friday ... while attaching several photos of the father and daughter enjoying past times together.
"Happy 13th birthday Coco!" Velasquez said. "I miss you so much! Te amo mucho."
As part of his note, Velasquez also said he was hoping to be released from jail at a hearing on Friday.