Lump in Your Dog's Armpit: What It Could Be

Name: Lump in Your Dog's Armpit: What It Could Be
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-10-03

A dog lump in the armpit is most often a benign fatty tumor or a swollen lymph node, but the location matters because mammary tissue and lymph nodes sit in that area too. Some armpit lumps are harmless, and some are cancer. Only a veterinarian can tell the difference with an exam and testing.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

Armpit lumps are common in older dogs, and fatty tumors (lipomas) are the most frequent benign cause [1].
The armpit holds lymph nodes and, in female dogs, mammary tissue, so a lump there has a wider list of possible causes than a lump on the back.
A lump that is firm, fixed to the tissue underneath, growing fast, or painful needs a veterinary visit soon.
Your vet will likely use a fine needle aspirate first, which is quick and often done during the exam.
Do not squeeze, poke, or try to drain the lump at home.

What an Armpit Lump Looks Like and What Is Normal

The armpit (also called the axilla) is the hollow area behind the front leg where it meets the chest wall. This region is packed with lymph nodes, blood vessels, nerves, and connective tissue. In female dogs, it also sits near the mammary chain.

A normal armpit feels soft and even. You may feel a small, pea-sized lymph node, but it should be smooth and movable. What counts as abnormal:

A new bump you can see or feel
A lump that has grown since you last checked
A firm or hard mass
A lump that feels stuck to the skin or the chest wall
Warmth, redness, pain, or hair loss over the area
A lump that drains fluid or bleeds

Dogs do not always show pain with lumps. Many dogs with serious armpit masses act completely normal at first. That is why size, texture, and growth rate matter more than how your dog seems to feel.

Main Causes of a Lump Under a Dog's Armpit

Skin tumors are the most commonly seen tumors in dogs, partly because they are easy to spot and partly because skin is exposed to things like chemicals, solar radiation, and viruses that can trigger abnormal cell growth [2]. Hormones and genetics can play a role too [2].

Here is a breakdown of the most likely causes when the lump is in the armpit.

Cause	Typical clues	How urgent
Lipoma (fatty tumor)	Soft, squishy, movable, slow growing, common in older dogs [1]	Routine visit
Swollen lymph node	Firm, bean-shaped, may be tender, often with infection or inflammation nearby	Visit within days
Mammary tumor	Firm nodule near the nipple chain, more common in unspayed females	Visit within days
Mast cell tumor	Variable feel, may change size, can be itchy or red	Visit within days
Abscess or infection	Warm, painful, may drain pus, dog may run a fever	Same day
Seroma or fluid pocket	Soft, fluid-filled, often after trauma or surgery	Routine to same day
Other soft tissue tumor	Firm, fixed, fast growing, may not be movable	Same day to urgent

Lipomas

Lipomas are benign fatty tumors and are common in older dogs [1]. They usually feel soft and squishy, move freely under the skin, and grow slowly. Most are harmless. A lipoma in the armpit can still cause problems if it grows large enough to press on nerves or limit leg movement. Rarely, fatty tumors form deep inside the body, and those are harder to find and treat [1].

Swollen Lymph Nodes

The armpit contains the axillary lymph nodes. These nodes filter fluid from the front leg and chest area. When they swell, it can mean the body is fighting an infection, inflammation, or cancer. A swollen lymph node often feels firm and bean-shaped. It may be tender.

Lymph node swelling can also be caused by cancer that starts in the node itself. One case report described a mast cell tumor arising primarily in a lymph node of a dog, with no skin tumor found anywhere on the body [3]. This is rare, but it shows that a swollen lymph node is not always just a reaction to infection [3].

Mammary Tumors

In female dogs, the mammary chain extends from the chest to the groin. The front pair of mammary glands sits near the armpit. A firm nodule in this area could be a mammary tumor. These are more common in dogs who were never spayed or were spayed later in life. Mammary tumors can be benign or malignant, so any firm nodule near the nipple chain deserves a veterinary exam.

Mast Cell Tumors

Mast cell tumors are a common skin cancer in dogs. They can look like almost anything: a small bump, a red patch, a wart, or a soft lump. They can change size quickly, sometimes shrinking and swelling within days. Because they release histamine and other chemicals, they may cause redness, itching, or swelling around the lump. A mast cell tumor in the armpit needs prompt evaluation because the area is hard to remove with wide margins.

Abscesses and Infections

An abscess is a pocket of pus, often from a bite, scratch, or foreign object. It feels warm and painful, and your dog may resist you touching it. Some abscesses drain on their own. Others need to be lanced and flushed by a veterinarian. Your dog may also need antibiotics. If your dog has a fever, is limping, or the area is hot and swollen, call your vet the same day.

Other Causes

Less common causes include cysts, seromas (fluid pockets after trauma or surgery), and other soft tissue tumors such as sarcomas. Some benign masses can look aggressive on imaging. One report described benign ectopic splenic tissue that mimicked malignant cancer on ultrasound in a dog [4]. This shows why a physical exam alone is not enough and why tissue sampling matters.

If you are seeing lumps in other areas too, our guide on Dog Skin Bumps and Lumps: When to See the Vet can help you decide what to do next.

Red Flags That Make It an Emergency

Call your veterinarian right away or go to an emergency clinic if you see any of these:

The lump is growing quickly (noticeably bigger in days)
The lump is hot, red, or draining pus
Your dog is limping or holding the leg up
Your dog has a fever, is lethargic, or is not eating
The lump is firm and fixed to the chest wall
Your dog seems painful when you touch the area
The lump bleeds without stopping
Your dog has trouble breathing or swallowing

A fast-growing lump in the armpit can press on nerves, blood vessels, or the airway. Do not wait to see if it goes away on its own.

What You Can Safely Do at Home While You Wait

You cannot diagnose or treat an armpit lump at home. But you can gather useful information for your vet.

Measure the lump with a soft tape measure and write down the size.
Take a photo with a ruler or coin next to it for scale.
Note when you first noticed it and whether it has changed.
Check for other lumps on the body, including the other armpit, groin, and along the mammary chain.
Do not squeeze, poke, or try to pop the lump.
Do not apply heat, ice, or human medications unless your vet tells you to.
Keep your dog from licking or chewing at the area. A cone may help.
Keep your dog calm and limit rough play if the leg seems sore.

If your dog is uncomfortable, call your vet about safe pain relief. Never give human pain medications like ibuprofen or acetaminophen to a dog. These can be toxic.

What Your Veterinarian Will Check and Test

Your vet will start with a full physical exam. They will feel the lump and compare it to the other side. They will also check the nearby lymph nodes, the mammary chain, and the rest of the skin.

Fine Needle Aspirate

This is the most common first test. Your vet inserts a thin needle into the lump, pulls out a few cells, and looks at them under a microscope. It takes only a minute or two and usually does not require sedation. The results can often tell the difference between fat, inflammation, and certain types of cancer.

Biopsy

If the aspirate is unclear or shows cancer, your vet may recommend a biopsy. A biopsy removes a piece of tissue for a pathologist to examine. After surgical removal, tumors should be evaluated under a microscope to confirm the type and whether all of the tumor was likely removed [2]. The pathologist checks the margins (the edges of the removed tissue) to see if tumor cells are present [2]. Clean margins mean tumor cells are far from the edge, and dirty margins mean tumor cells extend to the edge [2].

Imaging

X-rays or ultrasound may be needed to see how deep the lump goes and whether it is attached to the chest wall. Your vet may also check the chest for spread if cancer is suspected.

Blood Work

Blood work helps your vet assess your dog's overall health and rule out other conditions. It is often done before anesthesia or surgery.

Cytology and histopathology do not always agree. One study found that cytology and histopathology agreed in 55% of dogs with splenic masses, and disagreement occurred in 24% of cases [5]. This means a needle aspirate is a useful first step, but it is not perfect. A biopsy may still be needed for a clear answer.

If your dog has a lump that turns out to be a histiocytoma, a common benign skin tumor, you can read more about it in our article on Histiocytoma in Dogs: The Red Button Lump That Often Regresses on Its Own.

Prevention and Monitoring

You cannot prevent every lump, but you can monitor your dog's body regularly.

Do a monthly lump check. Run your hands over your dog's entire body, including the armpits, groin, and between the toes.
Spay your female dog if she is not intended for breeding. This reduces the risk of mammary tumors.
Keep your dog at a healthy weight. Obesity does not cause lipomas, but extra fat makes lumps harder to feel.
Take note of any lump that changes in size, shape, or texture.
Schedule annual vet visits for older dogs, and twice-yearly visits for seniors.

If you find a lump under your dog's skin and want a broader overview, see our comprehensive guide on Lump Under Dogs Skin: Comprehensive Veterinary Reference Guide.

Limitations and When to Contact a Veterinarian

This article cannot tell you what your dog's lump is. Only a veterinarian can examine the lump, run tests, and give you a diagnosis. Individual cases need a veterinarian.

Contact your veterinarian the same day if:

The lump appeared suddenly
Your dog is limping or seems painful
The lump is warm, red, or draining
Your dog has a fever or is not eating

Go to an emergency clinic now if:

The lump is growing very fast
Your dog is having trouble breathing
The lump is bleeding and will not stop
Your dog collapses or seems weak

Schedule a routine visit if:

The lump is small, soft, and slow growing
Your dog is acting normal
You first noticed it weeks ago and it has not changed

When in doubt, call your vet. It is better to have a benign lump checked than to miss a treatable cancer.

Frequently Asked Questions

Is a lump in my dog's armpit always cancer?

No. Most armpit lumps are benign, and lipomas are the most common fatty tumors in older dogs [1]. However, the armpit also contains lymph nodes and mammary tissue, so cancer is possible. A vet visit with a needle aspirate is the only way to know.

Can I tell if the lump is a lipoma by touching it?

Sometimes. Lipomas often feel soft and squishy and move easily under the skin [1]. But mast cell tumors and other cancers can feel soft too. You cannot rely on feel alone.

What does a swollen lymph node in the armpit mean?

It usually means the body is reacting to infection or inflammation in the area. It can also mean cancer. A lymph node that stays enlarged or feels firm should be checked by your vet.

Should I wait and see if the lump goes away?

If the lump is small, soft, and your dog is acting normal, you can monitor it for a few days. If it grows, changes, or your dog seems uncomfortable, call your vet. Do not wait weeks for a firm or fast-growing lump.

How much does it cost to remove an armpit lump?

Costs vary widely based on your location, the size of the lump, and whether anesthesia or imaging is needed. Your veterinarian will give you an estimate after examining your dog.

References

Perirenal lipoma in a 14-year-old spayed female dog: A rare diagnosis. The Canadian veterinary journal = La revue veterinaire canadienne, 2026.
Tumors of the Skin in Dogs - Dog Owners. Merck Veterinary Manual.
Suspected canine primary lymph node mast cell tumor. The Canadian veterinary journal = La revue veterinaire canadienne, 2025.
Ectopic Splenic Tissue Mimicking Malignant Neoplasia in a Dog. Veterinary radiology & ultrasound : the official journal of the American College of Veterinary Radiology and the International Veterinary Radiology Association, 2026.
Cytology and histopathology have poor to fair agreement for determination of neoplastic or nonneoplastic lesions in dogs with splenic masses or nodules. Journal of the American Veterinary Medical Association, 2026.