Only service dogs are allowed in U.S. post offices. Post offices are federal buildings, and within federal buildings, state laws have no jurisdiction. Federal law prohibits dogs in post offices, but the Americans With Disabilities Act (ADA), creates an exception for service dogs. There is sometimes confusion about state jurisdiction and licensing of service dogs. The ADA allows service dogs that meet ADA definitions, regardless of state licensing.
No Dogs Allowed, Except Service Dogs
The U.S. Postal Service is much like most businesses in this regard. Post office users have no way to determine whether a dog can safely interact with people. Sometimes even the best dog owners forget that their well-socialized dog may cause anxiety in other people. Still other dog owners fail to discern whether their dog is safe in public. As a result of these and other factors, such as hygiene, dogs are not permitted in offices of the USPS, with the exception of ADA rules.
You may be most familiar with trained seeing-eye dogs that assist with people who have sight impairments. But service dogs can help people with a range of disabilities. A dog trained to do a task a person cannot do because of their disability is a service dog that becomes a surrogate for person's ability. Service dogs are not limited to people who are seeing- and hearing-impaired.
Certification of Service Dog
Most states have training and certification programs for service dogs. This is sometimes a point of confusion for service dog rules in post offices. Service dogs have been denied access because they didn't appear to be service dogs in any official capacity. The incorrect reasoning is that if a state has not certified a dog as being a service dog, it's not allowed in the building. In fact, any dog trained to compensate for a person's disability is considered a service dog by the ADA, which grants them access to post offices. While it is not a requirement, it can be helpful to register your service dog.
Service Dog Constraints
Another point of confusion for service animals is the need for constraints, such as a leash or harness. The post office cannot require constraints. Not all service animals can do their job while wearing constraints, so requiring them could effectively eliminate the benefit of the service dog to its person.
There is only one exception to the ADA. If a dog exhibits an out-of-control or aggressive behavior, post office employees can require the dog to leave the premises. A way to remember this rule is to think of the dog as part of the person. You cannot ask a part of a person to leave a post office. However, if a person is disruptive, out of control or aggressive, the post office and most other public places have the right to insist they leave.