Why Does My Dog Have a Blue Mark on Their Lower Belly?

Any strays, or previously owned dogs, tend to have a small tattoo on their stomach to show they’ve been neutered or spayed. This is done primarily to confirm with certainty that there aren’t any reproductive organs to remove.

It serves a purpose: vets can visually confirm at a surface level instead of costly scans or invasive surgeries to double-check the animal is missing the organ(s).

Yes, even male dogs can have hidden organs, so they get the tattoo too. These tattoos won’t scrub off, by the way, so don’t try. They should function the same as ink in human skin—flat, smooth, and perhaps a bit faded, depending on your pet’s age. You’ve likely never noticed them before!

Pet owners tend not to look too closely at the area, while new fur parents might not have a reference point to know about the tattoo, so noticing it for the first time can come as a surprise.

Even my cat has one at the back half of his inner thigh. It’s a common practice to minimize risks to the animal’s health and helps reduce overbreeding between strays. If your dog allows, next time you’re rubbing their belly, you can check to see if they have the mark.

Fun fact: Some dogs may also have a mark on their ear. Others, such as street dogs and cats, may have alterations to their ear to clearly indicate they’ve been sterilized. All forms act as a professional language to communicate between offices, which proves particularly useful when a dog is found as a stray or a runaway.

However…

Should You be Worried?

If you’re concerned, you should check in with your vet.

You know your dog better than a blog post or article, and if their behaviors are off… go! Trust your gut instinct; it’s better to know it’s a vet-administered mark than to assume it is one and be wrong.

There are a few instances of blue in dogs where you should absolutely go to the vet. Blue tongues, gums, and skin (such as on the stomach!) can indicate issues with oxygen in the blood, according to this Hill’s Pet article by Dr. Laci Shaible.
So, when in doubt, go to the vet.