Attire that is not appropriate for the theme parks would include costumes (for Guests 14 years of age or older), clothing with objectional material, including obscene language or graphics, excessively torn clothing, or loose-fitting clothing which may drag on the ground and create a potential trip hazard, and clothing ...

For adults (anyone 14 and older), the answer is generally no, you cannot wear a full character onesie to Disneyland during normal operating hours. Disney's official 2026 dress code strictly prohibits adults from wearing costumes or clothing that could be viewed as representative of an actual Disney character, as this can cause confusion for children and safety issues for official "Cast Members." However, there are two exceptions: First, you can wear onesies during specific ticketed events like "Mickey's Not-So-Scary Halloween Party," provided they don't have masks that cover the face. Second, you can participate in "Disneybounding," which involves wearing everyday clothes (like a onesie that is just a solid color or a subtle pattern) that hints at a character without being a literal costume. A simple, non-character animal onesie might be allowed at the discretion of security, but if it looks like you are "portraying" a Disney character, you will likely be asked to change.