Broccoli — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
Safe in small pieces (steamed is gentlest). Stems in bulk can cause gas; the florets contain isothiocyanates that can upset the stomach in large amounts.
Short answer
Yes, in small amounts. Steam it, chop it finely, and offer only a few tiny pieces as an occasional treat. Too much stem can cause gas, and the florets contain isothiocyanates that may upset your cat's stomach if fed in large quantities. Watch for vomiting or diarrhea, and call your vet or the ASPCA Poison Control line (888-426-4435) if symptoms develop.