Keep the dog safe, get it scanned for a microchip, report it to animal control and post a found notice that leaves one detail out.
The short answer
If you found a dog with no collar, make sure it is safe, have it scanned for a microchip at a vet or shelter, and report it to your local animal control or shelter the same day. Then post a found notice near where you found it and in local groups, leaving one detail out so you can verify any owner. Do not assume no collar means no owner. Collars come off, and many lost dogs are simply frightened family pets. Rules on holding and keeping a found dog vary by city and county, so ask your local shelter what applies before you make any plans.
Keep yourself and the dog safe
Approach slowly and from the side, and do not reach over its head. A frightened or injured dog can bite, even a friendly one. Offer water and a little plain food. If it shows signs of fear, do not corner it. Use a leash or a looped rope when you can. If the dog is on or near a highway or will not let you approach, call animal control rather than chasing it into traffic. If it is hurt, go straight to a vet or the shelter. Oakland, California, for example, tells finders to take injured animals to the shelter immediately.
Get it scanned
A microchip is often the fastest route home, and it is invisible. Any vet office or shelter can usually scan a dog for free, though call first and ask. A chip number leads to a registry, not directly to an owner, and the AAHA lookup shows which registry holds it. Let the vet or shelter contact the registry; that is the normal process. If you are told there is no chip, ask them to check again across the back and shoulders. Also look inside the ears and on the belly for a tattoo, and check for a rabies tag or a mark from a collar.
Report it, even if you plan to care for it
File a found report with your local animal control and with any shelter near where you found the dog. The owner will probably go there first. Oakland says shelters are usually the first place owners look, and asks finders to file reports at local shelters within a 25-mile radius. Ask whether you can legally hold the dog at home during the hold period. Some places allow this, and may call it a finder foster, while others want the animal brought in. Keep a record of the date, the person you spoke with and the report number.
Understand the hold period
Most places set a stray hold, a period for an owner to come forward. It differs by area and often by whether the dog has identification. Oakland uses four business days including a Saturday without ID and 10 days with it. Lee County, Florida lists three days without ID and five with. Chatham County, Georgia sets a five-day hold. Your local rule may be different, so ask. Do not assume you can keep a dog because no one called within a few days; ask how the process works. In some places, a finder can apply to adopt at the end of the hold.
Post a found notice
Put up found dog flyers near where the dog was found, at corners, vets, pet stores and shops. Post in local lost and found groups and on Nextdoor. Oakland advises posting flyers near the spot where the animal was found. Check for lost posters in the area as well. Keep your wording simple: FOUND DOG, the area, the date and your phone number. Do not list every marking, and do not give the dog’s sex if you can avoid it. Stay with one photo, ideally without the most distinctive feature clearly visible. Put your cross streets on the flyer and keep your home address off.
Verify anyone who claims the dog
Most people who call will be real and desperate, but not all. Ask them to describe a marking you left off the notice, to send several photos from before the dog went missing, or to give vet records with the same chip number. Watch how the dog behaves around them, and hand over only at a shelter, a vet clinic or a similar public place. If someone asks you for personal details or wants a dog shipped elsewhere, slow down. If you have doubts, ask the shelter to handle the reunion. Do not give out the dog to the first person who calls.
If nobody comes forward
Keep your flyers up through the whole hold period and tell the shelter you are still looking. Dogs can travel, so widen the area. Keep your notes. If the hold period ends and the dog is still with you, ask your shelter about the proper steps to adopt or rehome it, including vaccinations and licensing. A vet visit is sensible early on, both for the dog’s health and to keep any other pets you have safe.
Sources
Oakland Animal Services: oaklandca.gov/Community/Pets-Animals/Lost-and-Found. Lee County Animal Services: leefl.gov/animalservices/lostpet/foundpet. Chatham County Animal Services: animalservices.chathamcountyga.gov/Services/FoundPets. AAHA microchip lookup: aaha.org/petmicrochiplookup. General information, not legal advice. Check your local rules.