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To evaluate an appropriate supplementation level, cassava starch-processing residue (CR) was added to liquid diets for growing-finishing pigs at 0, 3, or 6% of the basal-feed weight. A total of 480 pigs (initial body weight, 34.6 ± 0.46 kg) were fed the experimental diets for 12 weeks. Feed intake increased linearly during weeks 1–3, 4–6, and 7–9 (p < 0.05) and tended to increase over the entire feeding period (p = 0.057). Average daily gain decreased linearly only during weeks 7–9 (p = 0.013), whereas overall average daily gain and final body weight were unaffected. In contrast, the gain-to-feed ratio decreased linearly during weeks 1–3, 4–6, 7–9, and overall (p < 0.05). Increasing CR supplementation also linearly reduced the apparent total tract digestibility of dry matter, ether extract, and crude ash (p < 0.05), without affecting nitrogen digestibility. At week 12, fecal acetate increased linearly (p = 0.025), while total short-chain fatty acids tended to increase (p = 0.066). Muscle crude fat, oleic acid, and total monounsaturated fatty acids increased linearly, whereas cooking loss decreased (p < 0.05). Overall, increasing CR supplementation maintained overall growth but progressively reduced feed efficiency and the digestibility of dry matter and fat. Under the conditions of this study, 3% CR may represent a practical supplementation level when maintenance of growth performance is the primary consideration, although some reduction in feed efficiency should be expected; increasing CR supplementation to 6% resulted in greater reductions in feed efficiency and nutrient digestibility without additional benefits to growth.