Background: Since insulin resistance is thought to be the underlying mechanism for metabolic syndrome, affected individuals might be sensitive to a dietary sodium intervention. We aimed to examine the association between metabolic syndrome and salt sensitivity of blood pressure. Methods: 1906 Chinese participants without diabetes, aged 16 years or more, were selected to receive a low-sodium diet (51·3 mmol per day) for 7 days followed by a high-sodium diet (307·8 mmol per day) for an additional 7 days. Participants were excluded from the analysis if metabolic risk factor information was missing or if they did not complete their dietary interventions. Blood pressure was measured at baseline and on days 2, 5, 6, and 7 of each intervention. Metabolic syndrome was defined as the presence of three or more of: abdominal obesity, raised blood pressure, high triglyceride concentration, low HDL cholesterol, or high glucose. High salt sensitivity was defined as a decrease in mean arterial blood pressure of more than 5 mm Hg during low-sodium or an increase of more than 5 mm Hg during high-sodium intervention. This study is registered with ClinicalTrials.gov, number NCT00721721. Findings: Of the 1881 participants with information regarding metabolic syndrome, 283 had metabolic syndrome. 1853 participants completed the low-sodium diet and 1845 completed the high-sodium diet. Multivariable-adjusted mean changes in blood pressure were significantly greater in participants with metabolic syndrome than in those without on both low-sodium and high-sodium diets (p<0·0001 for all comparisons). Additionally, risk of salt sensitivity rose with increasing numbers of risk factors for metabolic syndrome. Compared with those with no risk factors, participants with four or five had a 3·54-fold increased odds (95% CI 2·05-6·11) of high salt-sensitivity during the low-sodium and a 3·13-fold increased odds (1·80-5·43) of high salt-sensitivity during the high-sodium intervention. Interpretation: These results suggest that metabolic syndrome enhances blood pressure response to sodium intake. Reduction in sodium intake could be an especially important component in reducing blood pressure in patients with multiple risk factors for metabolic syndrome. Funding: National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD, USA.