Radishes contain some vitamins and minerals that could be beneficial in very small amounts.
Risks
Radishes are quite spicy and fibrous, which can cause gastrointestinal upset in cats, including vomiting and diarrhea.
Suggestions
If you decide to give your cat radish, ensure it is in very small, finely chopped amounts and monitor for any adverse reactions.
Note: even if labeled as safe, any food can potentially cause allergies. When introducing a new food, please feed a small amount and observe your cat for any allergic reactions.