The redness of the knee may be caused by the following: 1. It may be caused by the thinness of the patient’s skin here. If the patient’s skin is too thin, it is prone to redness in some raised areas, which is more obvious in hot weather and after strenuous activities. 2. It may be caused by some inflammation inside the knee joint. When gouty arthritis or rheumatoid or rheumatoid arthritis strikes, there may be redness, swelling, and heat on the surface of the knee joint, accompanied by significant knee pain and limited movement. 3. It may be caused by local infection. Infection in the soft tissues around the knee joint or an infected lesion in the joint cavity may cause redness, swelling, and heat on the surface of the knee joint. 4. It may be caused by trauma to the knee joint. This is most commonly caused by friction or extrusion, which leads to localized subcutaneous congestion, resulting in redness of the knee joint.