College student Ronald Gjoka was arrested by police after invading the pitch during a Madrid clash to hug the Portugal captain.
After the young fan spent a day in jail in the summer, Ronaldo's letter, which has been published by Marca, has expressed concerns that any criminal record could harm his future.
In part of the letter, Ronaldo said: "As I understand it, he has admitted his mistake and understands the importance of security. He is a young man of 20 who grew up without a father and was raised by a single mother. He attends Palm Beach State Community College, Florida, with an international student grant.
"I am concerned because I understand he may face problems with immigration and with his college if the criminal charges against him are not dropped.
"He was immediately taken to the stadium's security area and then spent a day and a half in jail. It seems he appeared before a judge on Monday accompanied by his attorney who asked for the charges to be dropped, but you chose to ignore his appeal.
"I understand your position and the importance of enforcing rules and laws. However, I respectfully request that you and your office reconsider this decision and dismiss the two criminal charges this young man faces."
Gjoka said after his arrest that meeting his "idol" was worth the detention.