Yes — Benadryl (diphenhydramine) and Tylenol (acetaminophen) work in different ways and are commonly taken together, but stick to the dosing limits and ask your pharmacist, especially before mixing in any cough/cold combination products.
These two drugs do different jobs and are not expected to interact dangerously: diphenhydramine is a sedating antihistamine (for allergies, itching or sleep) and acetaminophen is a pain reliever/fever reducer. The NHS states directly that you can take diphenhydramine together with painkillers such as paracetamol (acetaminophen) or ibuprofen. The pairing is routine enough that the two ingredients are also sold premixed in nighttime "PM" products, so the main thing to manage is not a chemical clash but avoiding a double dose of either ingredient.
The two real risks are doubling up and drowsiness, not a chemical clash. (1) Avoid an accidental acetaminophen overdose: MedlinePlus advises not taking more than 4000 mg of acetaminophen per day from all sources, never taking more than one product that contains acetaminophen at a time, and reading every label — many nighttime, cold, flu and "PM" allergy products already contain acetaminophen and/or diphenhydramine, so if your Benadryl-type or cold remedy already has a painkiller in it, don't add separate Tylenol. Too much acetaminophen can cause liver damage serious enough to require a transplant or cause death. (2) Diphenhydramine causes marked drowsiness; both NHS and MedlinePlus warn not to drive or operate machinery until you know how it affects you, and to avoid alcohol because it adds to the sedation — use the same caution with other sedating drugs (opioids/codeine, sleep aids). Don't take acetaminophen if you drink three or more alcoholic drinks a day, or if you have liver disease, without talking to your doctor first. MedlinePlus notes diphenhydramine generally should not be used in older adults except for serious allergic reactions, as they are more prone to its sedation and confusion. When in doubt — or if you're pregnant, on other prescriptions, or treating a child — check with your pharmacist or doctor before combining.
This is general reference, not medical advice, and not a guarantee of safety. Interactions depend on your doses, health conditions, and other medicines. Always confirm with your pharmacist or doctor before combining products, and follow the dosing on each label.