A painful lump in your armpit is most often a swollen lymph node reacting to a nearby infection, an inflamed hair follicle, or a small abscess. These are the most common causes by far, and the majority resolve on their own or with simple treatment. Less commonly, a painful armpit lump can signal a skin condition called hidradenitis suppurativa or, rarely, something more serious that needs medical evaluation.
Your armpit is a high-traffic zone: it contains dozens of lymph nodes, dense clusters of hair follicles, and plenty of sweat glands, all packed into skin that rubs against itself constantly. That combination makes it one of the most common places on the body for lumps to appear.
Swollen Lymph Nodes
The most likely explanation for a tender armpit lump is a swollen lymph node. You have roughly 20 to 40 lymph nodes in each armpit, and their job is to filter bacteria, viruses, and other threats from the fluid draining your arm, chest wall, and breast tissue. When your immune system ramps up to fight an infection, nearby lymph nodes swell and can become sore to the touch.
Common triggers include a cut or skin infection on your hand or arm, a cold or upper respiratory infection, or even a recent vaccination (COVID and flu vaccines frequently cause temporary armpit swelling on the injected side). Reactive lymph nodes are typically oval-shaped, smaller than about 2 centimeters, somewhat movable under the skin, and tender. They usually shrink back to normal within one to two weeks once the underlying infection clears.
Infected Hair Follicles
Shaving, waxing, and tight clothing can all damage hair follicles in the armpit, creating tiny openings where bacteria enter. The result is folliculitis: small, red, painful bumps that may look like pimples and sometimes fill with pus. People with curly hair are especially prone to ingrown hairs after shaving, which can mimic infected follicles even without bacteria being involved.
If you shave your armpits regularly, a few habits can reduce flare-ups: shave in the direction of hair growth, use a clean and sharp blade, avoid stretching the skin taut, and apply a gentle moisturizer afterward. Washing the area with warm water and a mild cleanser before shaving also helps. Switching to an electric razor or trimmer that doesn’t cut as close to the skin can make a noticeable difference if you get these bumps frequently.
Abscesses and Cysts
An abscess is a pocket of pus caused by a bacterial infection. In the armpit, it typically appears as a soft, raised lump that is red, warm, and painful. Abscesses often develop from an untreated infected follicle or a small wound that traps bacteria beneath the skin. They tend to grow over several days, becoming increasingly tender and sometimes causing fever.
A cyst, by contrast, is a firm, enclosed sac that can contain fluid, air, or a thick cheese-like substance. Cysts in the armpit are usually painless unless they become infected, at which point they behave a lot like an abscess. The key difference you can feel: abscesses are soft and squishy, while cysts are firmer and more defined. Small abscesses sometimes drain on their own, but larger ones often need to be drained by a healthcare provider.
Hidradenitis Suppurativa
If your painful armpit lumps keep coming back, hidradenitis suppurativa (HS) is worth knowing about. This chronic skin condition typically starts as a single, painful pea-sized lump under the skin that persists for weeks or months. It develops in areas where skin rubs together, with the armpits being one of the most common sites.
Over time, HS can progress. Early stages involve recurrent tender bumps that fill with pus and small paired blackheads in pitted skin. Some bumps eventually break open and drain fluid with an odor. In more advanced cases, tunnels called sinus tracts form beneath the skin, connecting lumps to each other. These tunnels heal very slowly, if at all, and can cause significant scarring.
HS is not caused by poor hygiene. It involves the immune system and the hair follicles, and it tends to run in families. Signs that point toward HS rather than a one-time infection include lumps that appear in multiple locations (armpits, groin, buttocks, under the breasts), flares that recur in the same spots, and bumps that return within weeks of seeming to heal. A dermatologist can diagnose HS based on appearance and pattern alone, and early treatment can help slow progression.
Breast Tissue and the Armpit
Breast tissue actually extends into the armpit in a region called the tail of Spence. This means that lumps in the armpit can sometimes be related to breast health. Benign causes are far more common: a breast abscess, mastitis (especially during breastfeeding), or normal hormonal swelling before a period can all cause tender armpit lumps tied to breast tissue.
In rare cases, swollen armpit lymph nodes can indicate that a breast cancer has spread. Lymph nodes associated with malignancy tend to feel hard and fixed in place rather than soft and movable. On imaging, concerning features include a round shape (instead of the normal oval), thickened outer tissue, and loss of the fatty center that healthy lymph nodes have. These characteristics are things your doctor evaluates with ultrasound, not something you can reliably assess by touch alone, which is why persistent armpit lumps warrant a professional exam.
What Your Doctor Will Check
A physical exam is usually the first step. Your provider will note the lump’s size, shape, firmness, and whether it moves freely under the skin. They will also check for signs of infection like redness, warmth, or fever, and ask about recent illnesses, injuries, vaccinations, or shaving habits.
If the lump doesn’t have an obvious explanation, an ultrasound is the most common next step. Ultrasound can distinguish between a fluid-filled cyst, an abscess, and a solid mass, and it reveals the internal structure of lymph nodes in detail. If imaging shows anything unusual, a needle biopsy guided by ultrasound can sample tissue from the node without surgery. In many cases, though, the ultrasound alone is enough to confirm a benign cause and no biopsy is needed.
When a Lump Needs Medical Attention
Most armpit lumps from shaving irritation or a mild infection resolve within a week or two. You should see a doctor if the lump:
- Lasts longer than two weeks without shrinking
- Gets bigger over time
- Feels hard and does not move when you press on it
- Comes with a fever or spreading redness
- Returns after it seemed to go away or was previously removed
- Appears alongside unexplained weight loss, night sweats, or fatigue
A lump that is soft, movable, appeared after shaving or during a cold, and is already getting smaller is very likely harmless. One that is growing, painless and rock-hard, or accompanied by systemic symptoms like persistent fever deserves prompt evaluation.
Home Care for Mild Lumps
For a lump that seems related to a minor infection or irritated follicle, applying a warm, damp compress for 10 to 15 minutes several times a day can help draw pus toward the surface and relieve pain. Keep the area clean, avoid shaving over the lump, and wear loose clothing that doesn’t press against it. Over-the-counter pain relievers can manage discomfort while you wait for the lump to resolve. Avoid squeezing or trying to pop it, since that can push bacteria deeper into the tissue and worsen the infection.