Background/Objective Behçet disease (BD) is not a single unique entity but a syndrome with different clinical phenotypes that can involve arterial and venous vessels of all sizes. To date, there has been no specific test or serum marker to measure and determine the severity of BD, and diagnosis remains based on clinical findings. This study aimed to assess lower extremity venous wall thickness (VWT) measured by ultrasound and laboratory findings and diagnostic performance in patients with BD. Methods A total of 106 participants were recruited from the rheumatology department in this single-center, case-control study. Participants meeting the eligibility criteria were divided into healthy controls (n = 52) and BD (n = 54). The VWT values of the common femoral vein, great saphenous vein, and popliteal vein were measured using ultrasonography. Laboratory data were obtained from the electronic registration database. Venous wall thicknesses and laboratory findings in patients with BD and healthy subjects were compared. Results Venous wall thickness of the lower extremity veins was higher in the BD group and higher in those with a history of deep vein thrombosis than in those without. The mean leukocyte, monocyte, erythrocyte sedimentation rate (ESR), C-reactive protein, plateletcrit (PCT), red cell distribution width (RDW), mean platelet volume (MPV) values, and monocyte-to-lymphocyte ratio (MLR) were higher in BD patients than in the control group. There was a correlation among increased VWT, ESR, PCT, MPV, RDW, and MLR. Conclusions C-reactive protein, ESR, MPV, PCT, MLR, RDW, and VWT can be used to assist in the diagnosis of BD.