This study investigated the use of basil seed gum (BSG) as a fat substitute on the physicochemical properties and antioxidant activities of yogurt. A 0.5 and 1% BSG solution was supplemented to reduced-fat and nonfat yogurts, and their physicochemical properties, quality properties, antioxidant activity, and sensory evaluation were compared with each control group. We prepared 3 yogurts as controls and 4 yogurt samples containing BSG as follows: FFY (yogurt made from full-fat milk: a control group), LFY (yogurt made from reduced-fat milk: a control group), SY (yogurt made from nonfat milk: a control group), LFY 0.5% (0.5% BSG added to reduced-fat yogurt), and LFY 1.0% (1.0% BSG added to reduced-fat yogurt), SY 0.5% (0.5% BSG added to nonfat yogurt), and SY 1.0% (1.0% BSG added to nonfat yogurt). The pH of LFY 0.5% and LFY 1.0% was decreased compared with LFY control, whereas pH of SY 0.5% and SY 1.0% had no significant difference. The titratable acidity showed no significant increase. The viscosity was the highest in FFY among the control groups and increased with the concentration of BSG in the SY group. The L-value (brightness) and b-value (yellowness) were the highest in FFY at 85.05 among the control groups. The L-value and b-value of LFY 0.5% and SY 0.5% showed higher values than LFY 1% and SY 1%. The a-value (redness) was the highest in SY 0.5% at -2.36, and ΔE (total color difference) was the highest in SY 1% at 7.33. The moisture content of SY was the highest among the control groups and addition of 1% BSG to SY was highest among the BSG-added group. Total contents of phenol and flavonoid slightly increased as the concentration of BSG increased (increase in the contents of phenol and flavonoid). The results of ferric reducing antioxidant power were similar to the findings of phenol and flavonoid content (an increase as the concentration of BSG increased). The overall acceptability of sensory characteristics was improved in all groups of samples when BSG 1% concentration increased. Application of BSG for the production of nonfat yogurt can enhance physicochemical properties, antioxidant activity, and sensory characteristics of reduced-fat and nonfat yogurt. Addition of BSG to reduced-fat and nonfat yogurt can improve their physical and antioxidant properties to the level of FFY.