When only one knee is swollen, doctors often call it a unilateral knee effusion or “water on the knee,” meaning extra fluid has collected in or around that single joint. The “normal” knee is actually very useful for comparison, because it helps clinicians see how much bigger, warmer, or stiffer the affected knee truly is.

Common Causes When Only One Knee Is Swollen

Even when only one knee is puffy or tight, many of the usual swollen‑knee causes still apply. Some are relatively mild and self‑limited, while others are serious and need quick intervention.

1. Injury or Overuse

Sudden twisting, a fall, or a direct blow can damage ligaments (like ACL or PCL), cartilage (meniscus), or bone and trigger rapid fluid buildup. This can cause significant swelling, stiffness, and sometimes bruising around just one knee.
Overuse—such as long hikes, new workout routines, or frequent kneeling—can irritate the joint lining or bursae, producing gradual swelling and discomfort without a clear “big injury” moment.

2. Osteoarthritis (“Wear and Tear” Arthritis)

Osteoarthritis happens when the cartilage that cushions your knee gradually breaks down; it often affects one knee more than the other, especially if one side has carried more load over the years.
Flare‑ups of pain, stiffness, and mild‑to‑moderate swelling can make one knee look and feel “old” while the other still seems relatively normal, particularly after activity or at the end of the day.

3. Inflammatory Arthritis and Autoimmune Conditions

Conditions like rheumatoid arthritis, psoriatic arthritis, or other inflammatory arthropathies can sometimes start as monoarthritis, meaning pain and swelling in just one joint, including a single knee.
These conditions often cause warmth, morning stiffness, and low‑grade systemic symptoms (fatigue, mild fever), and may later involve other joints over time if not treated.

4. Crystal Arthritis (Gout or Pseudogout)

Gout occurs when uric acid crystals deposit in a joint, while pseudogout involves calcium pyrophosphate crystals; both can strike suddenly and dramatically in one knee.
People often describe intense pain, a very swollen, hot, red knee, and difficulty bearing weight, while the other leg looks completely fine.

5. Bursitis Around the Knee

The knee has several bursae—small fluid‑filled sacs that reduce friction between bone and soft tissues—and inflammation of these (for example, prepatellar bursitis from kneeling) can cause localized swelling in one knee.
Bursitis swelling is often more superficial, with a visible “pocket” of fluid at the front or side of the knee, and it may be tender to touch or linked to specific positions or activities.

6. Infection (Septic Arthritis or Infected Bursa)

A joint infection is one of the most serious causes of one swollen knee; it can destroy cartilage quickly and is considered an orthopedic emergency.
Septic arthritis typically causes marked swelling, intense pain, warmth, redness, and often fever or feeling unwell, and needs urgent hospital‑level evaluation and antibiotics.

7. Less Common but Important Causes

Tumors, bleeding disorders, or conditions like hemarthrosis (bleeding into the joint after significant injury or in people on blood thinners) can all present as a markedly swollen single knee.
Some systemic infections (like Lyme disease) or autoimmune diseases can first draw attention because one knee becomes noticeably swollen and stiff.
One Knee Swollen, Other Normal

When One Swollen Knee Means You Should Be Worried

Even if only one knee is involved, certain red‑flag signs suggest that waiting it out at home is not safe. These warning features focus on the speed, severity, and systemic nature of your symptoms.

Red Flags Requiring Urgent or Emergency Care

Seek emergency or immediate medical help if you notice any of the following with your swollen knee:

Sudden, severe pain that makes it hard or impossible to bear weight on that leg
Very rapid swelling within a few hours of an injury, especially with deformity or inability to straighten the knee (possible fracture, major ligament tear, or hemarthrosis)
Obvious deformity, the knee looking “out of place,” or a locked joint that will not move
Warmth, redness, and severe tenderness over the joint, especially if you also have fever, chills, or feel unwell (possible septic arthritis)
History of a weakened immune system, diabetes, recent surgery, a penetrating wound near the knee, or use of injected drugs combined with swelling and pain
One Knee Swollen, Other Normal

Concerning Signs That Need Prompt (But Not Emergency) Evaluation

Book a medical appointment soon—ideally within days—if any of these apply:

Swelling lasts more than a few days or keeps coming back in the same knee
Pain and stiffness are interfering with sleep, work, walking, or stairs even with rest and simple care
You notice the knee gradually becoming more bowed, unstable, or prone to “giving way” (possible progressive arthritis or ligament instability)
There is a history of gout, inflammatory arthritis, or knee surgery and this knee now behaves differently than usual flares

In these scenarios, early diagnosis can prevent further cartilage damage and guide targeted treatment instead of endless cycles of rest and painkillers.

What a Doctor Typically Checks for One Swollen Knee

Health professionals follow a structured approach to a single swollen knee to narrow down the cause. Comparing the swollen knee to the normal one is a key part of the assessment.

History and Physical Examination

The clinician asks when the swelling started, whether there was trauma, what activities or positions worsen it, and about systemic issues like fever, weight loss, or morning stiffness.
On examination, they look for warmth, redness, range of motion, instability, tenderness at specific points, and signs of fluid using maneuvers like the patellar tap, bulge sign, or balloon test.

Tests, Imaging, and Joint Fluid Analysis

Depending on findings, they may order X‑rays to look at bone and joint space, ultrasound to confirm effusion, or MRI to assess ligaments and cartilage.
In many cases of unexplained or severe swelling, the doctor may perform arthrocentesis—drawing fluid from the knee with a needle—to check for infection, crystals, or inflammatory markers, which can quickly distinguish gout, septic arthritis, and other causes.
One Knee Swollen, Other Normal

Home Care: When It Is Reasonable and How to Do It Safely

If there are no red flags—no major trauma, no fever, no severe or worsening pain, and you can bear weight—the cause may be a mild injury or overuse. In that context, short‑term home care can be appropriate while you monitor symptoms.

Practical Self‑Care Strategies

Rest and relative load management: Reduce or temporarily stop high‑impact activities like running, jumping, or deep squats; choose short walks on flat ground instead of complete bed rest to keep circulation and joint nutrition.
Ice and elevation: Applying a cold pack wrapped in cloth for about 15–20 minutes at a time and raising the leg above heart level can help limit swelling and discomfort in the early phase.
Compression: A soft elastic bandage or knee sleeve can provide gentle compression and support if not too tight and if there is no redness or severe pain.
Over‑the‑counter medication: Short courses of nonsteroidal anti‑inflammatory drugs (NSAIDs), if medically safe for you, may help with pain and inflammation, but they should not replace proper evaluation when symptoms are concerning.
One Knee Swollen, Other Normal

Everyday Choices to Protect a “Problem” Knee

If one knee tends to swell more than the other, even outside of an acute crisis, small lifestyle changes can reduce episodes and protect joint health.

Weight management: Excess body weight increases the load on the knees and is strongly linked to earlier and more severe osteoarthritis and effusions.
Strength and flexibility: Focused exercises to strengthen quadriceps, hamstrings, and hip muscles, plus gentle stretching, improve stability and may reduce flare‑ups of pain and swelling.
Activity modification: Choosing low‑impact activities such as walking on even ground, cycling, or swimming instead of repetitive jumping or running on hard surfaces reduces joint stress.
Footwear and supports: Supportive shoes and, when advised, orthotics or braces can help improve alignment and decrease uneven load on one knee.
One Knee Swollen, Other Normal

So, Should You Be Worried?

A single swollen knee should always prompt attention, but not always panic. Mild swelling after a clear, minor strain, with no red flags and steady improvement over a few days, is often manageable with simple home care and follow‑up as needed.

You May Know

Swollen Knee After Walking But No Injury

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One Knee Swollen, Other Normal