Background: The most common and crippling type of arthritis is osteoarthritis (OA), which is characterized as a degenerative disorder targeting synovial joints. Patients with osteoarthritis frequently lament a dull aching pain that worsens with movements. When physical agents as heat therapy are used properly, they can combat unpleasant conditions. Aim: This study was carried out to determine the effect of thermotherapy on Clinical Outcomes among Patients with Osteoarthritis. Design: A quasi-experimental research design (pre-post study/control) was used to achieve the study's aim Setting: The study was conducted in the orthopedic outpatient department of the shebin EL-Kom Teaching Hospital and Menoufia University Hospital, Menoufia Governorate, Egypt. Sample: 80 adults with unilaterally knee osteoarthritis were chosen as a convenient sample. Three different tools were used to collect data. Tool I: structured interview questionnaire. Tool II: Lysholm Knee Scoring Scale (LKSC). Tool II1: The Western Ontario and McMaster Universities Arthritis Index.  Results: The mean pain and stiffness scores were 13.62±2.10 and 5.40±0.92 respectively among the study group and 13.52±2.01 and 5.50±0.87 respectively among the control group, which were highly significantly decreased among the study group than the control group with P value = <0.001. Conclusions: A distinct effect of thermotherapy on knee osteoarthritis was observed in a form of decreasing pain, stiffness, and improving physical functions. Recommendation: Superficial heat therapy should be included in managing patients with osteoarthritis to relieve pain, stiffness and improve physical function. Moreover, replication of the study with a larger probability sample must be considered in the development of future research to allow for greater generalization of the results.