A lump on your knee is almost always benign. The most common causes are fluid-filled cysts, inflamed bursae (small cushioning sacs around the joint), and, in teenagers, a normal growth-related bump on the shinbone. Where the lump sits, how it feels, and whether it hurts all point toward different explanations.
Lumps Behind the Knee
The single most common knee lump is a Baker’s cyst, also called a popliteal cyst. It shows up on roughly 40% of knee MRIs and forms when excess joint fluid gets pushed into a small pocket behind the knee, between two muscles in the back of your leg. You’ll typically notice a soft, squishy bulge in the hollow behind the knee that becomes more visible when you stand with your leg straight and shrinks when you bend your knee to about 45 degrees.
Baker’s cysts don’t usually appear out of nowhere. In adults, they’re strongly linked to wear-and-tear changes inside the joint, particularly meniscal tears and arthritis. A torn meniscus can act like a one-way valve, pushing fluid out of the joint and trapping it behind the knee, where it thickens into a gel-like mass. Inflammatory conditions like rheumatoid arthritis also drive cyst formation by producing excess joint fluid.
Most Baker’s cysts cause a feeling of tightness or mild aching that gets worse with activity. If one grows large enough, it can press on nearby blood vessels and cause swelling in your lower leg. Occasionally a cyst ruptures or extends into the calf muscle, producing redness, swelling, and tenderness that can look almost identical to a blood clot. That scenario warrants prompt medical evaluation to rule out a deep vein thrombosis.
Lumps on the Front of the Knee
A soft, puffy swelling directly over your kneecap is the hallmark of prepatellar bursitis, sometimes called “housemaid’s knee” because it’s triggered by prolonged or repeated kneeling. Plumbers, gardeners, carpet layers, and anyone who spends time on their knees is at higher risk. The bursa, a thin fluid-filled sac that normally cushions the kneecap, becomes irritated and fills with extra fluid. You’ll see redness, feel warmth over the area, and notice tenderness when you press on it. Range of motion may be limited simply because bending hurts.
A similar bursa sits just below the kneecap, near the top of the shinbone. When that one flares up (infrapatellar bursitis), the swelling appears slightly lower. One study found deep infrapatellar bursitis in 41% of people with no knee symptoms at all, so a small amount of fluid in this area isn’t necessarily a problem.
The Teenager’s Bump: Osgood-Schlatter Disease
If you’re between roughly 9 and 13 years old, or you’re a parent noticing a hard, bony bump just below your child’s kneecap, the likely explanation is Osgood-Schlatter disease. It happens when the patellar tendon pulls repeatedly on the growth plate at the top of the shinbone during a growth spurt, causing pain, swelling, and a visible knob of bone. It’s especially common in kids who run, jump, or squat frequently. The bump tends to hurt with activity and feel better with rest, and it usually affects only one knee. It’s temporary. Most children outgrow it completely once the growth plate closes, though a small painless bump may remain permanently.
Lumps on the Sides of the Knee
The inner (medial) side of the knee has a bursa called the pes anserine bursa, which sits where three tendons attach just below the joint line. Irritation here causes a tender, soft swelling on the inner knee, often from overuse in runners or from osteoarthritis. On the outer (lateral) side, friction from the iliotibial band, a thick strip of connective tissue running down the outside of your thigh, can create fluid-filled, cyst-like swelling near the outside of the knee.
Ganglion cysts can also appear around the knee, most often toward the back or sides. These are filled with thick, jelly-like fluid and feel smooth and somewhat firm. Unlike meniscal cysts, which are always connected to a torn meniscus inside the joint, ganglion cysts form independently. They may have a connection to the joint capsule but don’t signal cartilage damage.
Hard, Bony Lumps
A lump that feels rock-hard and doesn’t move when you push on it is likely coming from the bone itself. The most common benign bone growth near the knee is an osteochondroma, a bony bump capped with cartilage that grows outward from the surface of the bone. The knee is one of the most frequent locations: the lower end of the thighbone accounts for about 30% of all osteochondroma cases, and the upper shinbone is the next most common site. These lumps grow slowly, are painless unless they press on a nearby tendon or nerve, and are typically discovered in childhood or adolescence.
When a Lump Needs Attention
The vast majority of knee lumps are harmless, but certain features should prompt a visit to your doctor sooner rather than later. A lump that keeps growing over weeks or months, particularly one that’s painless and deep, can occasionally be a soft tissue tumor such as a synovial sarcoma. The first sign is often just a lump under the skin that slowly gets bigger. Pain, joint stiffness, and swelling may follow, but many people notice the size change before anything else.
Other signals worth acting on: a lump that feels firmly fixed to deeper tissue and doesn’t slide under your fingers, rapid onset of redness and heat (which can indicate infection inside a bursa), calf swelling that mimics a blood clot, or any lump accompanied by unexplained weight loss or night pain.
How Knee Lumps Are Diagnosed
For a soft, superficial lump you can see and feel, ultrasound is usually the first step. It’s quick, inexpensive, and very good at distinguishing a fluid-filled cyst from a solid mass. If the lump appears to extend deeper into the joint or involves bone, MRI provides far more detail and is better at revealing associated problems like meniscal tears, cartilage damage, or connections between a cyst and the joint cavity.
Managing a Knee Lump at Home
If your lump appeared after a clear trigger like a day of kneeling, a long run, or a minor tweak, basic home care is reasonable for the first 72 hours: rest, ice for 15 to 20 minutes at a time, gentle compression with a wrap, and keeping the leg elevated. This approach works well for acute bursitis and minor swelling.
For Baker’s cysts, treating the cyst alone rarely solves the problem. Because the cyst is usually a downstream effect of something happening inside the joint (a meniscal tear, arthritis, inflammation), addressing that underlying issue is what keeps fluid from re-accumulating. A cyst that’s drained but whose root cause is ignored will typically refill.
If you’ve been managing a lump at home for more than a week or two with no improvement, or if it’s getting larger, more painful, or harder to move around, that’s a good signal to get it evaluated. Most knee lumps have straightforward explanations and respond well to targeted treatment once the cause is identified.