This study aimed to compare the effects of small-sided games (SSG) and high-intensity interval training (HIIT) on the physical health and fitness of sedentary, untrained young adults. A 16-week randomized controlled trial was conducted with 90 participants (mean age 19.82 ± 0.66 years; 45 males and 45 females), who were randomly assigned in a 1:1:1 ratio to either the SSG group, the HIIT group, or a non-training control group (CG). Each group included 30 participants, and all participants completed the intervention. All interventions were performed three times per week. The SSG group participated in 4v4 or 6v6 handball games, while the HIIT group completed 30-30 (30 s sprint/30 s active recovery) or 40-20 (40 s sprint/20 s active recovery) protocols on an outdoor rubber track at 80-85% of their maximum heart rate (HRmax). Measured outcomes included physiological indicators (body weight, body mass index [BMI], systolic/diastolic blood pressure [SBP/DBP], and resting heart rate [RHR]) and fitness indicators (grip strength, one-repetition maximum squat [1RM], 20-meter multistage fitness test [20 m MFT] performance, and maximal oxygen uptake [VO₂max]). Baseline characteristics were well balanced across groups (standardized mean differences < 0.50). After 16 weeks, both SSG and HIIT significantly reduced body weight, BMI, SBP, DBP, and RHR (p < 0.01), while significantly improving 1RM squat, 20 m MFT performance, and VO₂max (p < 0.01). In contrast, the CG group showed significant decreases in SBP, left-hand grip strength, and 1RM squat (p < 0.05). Between-group analyses indicated that SSG and HIIT produced substantially greater improvements than the CG in cardiometabolic outcomes, for example, VO₂max increased by approximately 7 mL·kg^-1·min^-1 in both training groups, whereas it showed little or no change in the CG. For body weight and BMI, only body weight differed significantly between the SSG and CG groups (p < 0.05). No significant between-group differences were observed in grip strength (p > 0.05), and no significant differences were found between the SSG and HIIT groups across any indicators (all p > 0.05). In conclusion, both SSG and HIIT effectively improved physical health and fitness in sedentary young adults. SSG may be more suitable for individuals seeking an enjoyable, game-based approach that supports upper limb strength development, while HIIT may be preferable for those aiming to enhance aerobic fitness efficiently. Because SSG offers an enjoyable, socially interactive, game-based format and HIIT provides a time-efficient, highly structured format, both modalities may promote sustained participation beyond initial adoption in previously inactive young adults.

• Both small-sided games (SSG) and high-intensity interval training (HIIT) significantly improved weight, BMI, blood pressure, resting heart rate, aerobic capacity, and lower-limb strength compared with the control group.

• SSG and HIIT produced similar overall health and fitness benefits, with no significant differences between them across most indicators.

• SSG showed stronger support for upper-limb strength, while HIIT provided slightly superior improvements in aerobic performance.