A ball-like lump under your armpit is most often a swollen lymph node, an inflamed hair follicle, or a cyst. The armpit is packed with lymph nodes, hair follicles, and sweat glands, so bumps in this area are common and usually not dangerous. That said, size, texture, and how long the lump sticks around all matter when figuring out what’s going on.
Swollen Lymph Nodes
Your armpits contain dozens of lymph nodes, small bean-shaped glands that filter fluid and trap bacteria and viruses. When your immune system is fighting something off, these nodes swell up and can feel like a firm, marble-sized ball under the skin. Normal lymph nodes measure less than 1 cm across (roughly the size of a pea). Once they grow larger than that, they’re considered abnormal.
The most common reason for a swollen armpit lymph node is an infection or injury somewhere in the arm, hand, or chest on that same side. A cut on your finger, a skin infection like cellulitis, or even a bad case of the flu can trigger it. Less common infections include cat scratch fever (from a cat scratch or bite) and certain fungal infections that enter through the skin. In most of these cases, the node shrinks back to normal once the infection clears.
Vaccine-Related Swelling
If you recently had a vaccination in that arm, the lump is very likely a lymph node responding to the vaccine. This is especially well documented after COVID-19 vaccines and boosters. Studies tracking post-vaccine armpit swelling found the average time to resolution was about 102 days, which is why current imaging guidelines recommend waiting at least 12 weeks before investigating further. The swelling is a sign your immune system is doing its job, not a sign something is wrong.
Ingrown Hairs and Folliculitis
Shaving is one of the most common reasons people develop a painful bump in the armpit. When a shaved hair curls back into the skin or a hair follicle gets infected with bacteria, you get folliculitis. It looks like a small pimple or pus-filled bump right around a hair follicle, and the skin around it is usually red, tender, and sometimes itchy or burning.
Mild folliculitis typically heals on its own within a few days. If you keep getting these bumps, a few changes help: shave less often, always shave in the direction of hair growth, use a clean sharp blade, apply shaving lotion beforehand, and moisturize afterward. Avoid sharing razors or towels. If a single inflamed follicle grows deeper, it can become a boil, a larger, more painful lump that fills with pus. A cluster of connected boils is called a carbuncle. These sometimes need medical drainage.
Cysts and Lipomas
A cyst feels like a round, somewhat firm ball under the skin. It may shift slightly when you press on it, but it often feels anchored in place. Some cysts have a tiny visible opening on the surface called a punctum. Inside, they’re filled with fluid or semi-solid material. Cysts are benign and often painless unless they become infected, at which point they turn red, swollen, and tender.
A lipoma is a slow-growing lump made of fat cells. It feels soft and rubbery, and the key difference is that it slides easily under the skin when you push on it. Lipomas are almost always harmless and painless. They don’t go away on their own, but they rarely need treatment unless they’re bothersome or growing.
Hidradenitis Suppurativa
If the lump has been there for weeks or months, keeps coming back, or you’ve noticed similar bumps in your groin or under your breasts, you may be dealing with hidradenitis suppurativa. This chronic skin condition starts as a single painful lump under the skin that persists much longer than a regular pimple would. It develops in areas where skin rubs together, and the armpit is one of the most common spots.
Over time, more bumps can appear. Some grow larger, break open, and drain pus that has a noticeable odor. In advanced cases, tunnels form under the skin connecting the lumps, and these wounds heal very slowly. Early treatment makes a significant difference in preventing progression, so recognizing the pattern early matters.
When the Lump Needs Attention
Most armpit lumps from infections or minor skin irritation resolve within one to two weeks. If a lump persists for three weeks or longer without shrinking, it warrants a medical evaluation. Size matters too: anything that grows beyond 1 cm, continues to enlarge, or feels hard and fixed in place (rather than soft and movable) raises more concern.
Hard or matted lymph nodes that don’t move freely can sometimes indicate a more serious cause. Breast, lung, skin (particularly melanoma), and several other cancers can spread to armpit lymph nodes. Lymphoma can also first appear as a painless, firm node in the armpit. The absence of an obvious infection or injury on the same side makes these possibilities more important to rule out. Importantly, a painless lump is not automatically reassuring, since malignant nodes often don’t hurt.
If your doctor wants a closer look, the first step is usually an ultrasound. It’s quick, inexpensive, and shows the internal structure of the node or lump in real time. A normal lymph node appears oval with a thin, even outer layer and a bright fatty center. Suspicious signs on ultrasound include a thickened or uneven outer layer, a round rather than oval shape, or loss of that bright center. If any of those features appear, a needle biopsy guided by ultrasound is the next step to get a definitive answer.
What You Can Do Right Now
Start by noting the basics: how big the lump is, whether it’s painful, how it feels when you press on it, and whether you’ve had a recent infection, injury, shave irritation, or vaccination on that side. A small, tender, movable lump that appeared after you nicked yourself shaving or came down with a cold is almost certainly nothing serious. Keep the area clean, avoid irritating it with deodorant or tight clothing, and give it a couple of weeks.
If the lump is larger than a grape, rock-hard, painless, growing, or has lasted more than three weeks, get it evaluated. The same goes if you notice multiple lumps, unexplained weight loss, night sweats, or similar bumps appearing in other body folds. These details help a clinician quickly narrow down what’s going on and decide whether imaging is needed.