(1) Background: Taxes on sugar-sweetened beverages are an effective public health intervention, but can be difficult to implement in the absence of public support. This is the first study to analyze the Spanish population's support for a tax on sugar-sweetened beverages. (2) Methods: We conducted a cross-sectional study of a representative sample of the Spanish adult population (n = 1002), using a computer-aided telephone interview with a questionnaire on nutritional policies. The support for the tax was calculated by the percentage of those who agreed plus those who strongly agreed with the measure. The sociodemographic determinants of support for the tax were analyzed using chi-squared test (chi(2)) and Poisson multiple regression models with robust variance. (3) Results: Of the participants, 66.9% supported a tax on sugar-sweetened beverages. Support for the tax was 9.2% higher (70% vs. 64.1%) when responders were first asked about support for tax relief and subsidies for healthy foods (p = 0.049). Support for the tax was 16% and 35% lower among persons reporting center and right-wing political sympathies (p < 0.01), and 16% lower among regular consumers of sugar-sweetened beverages (p = 0.01). (4) Conclusions: A clear majority of the Spanish population is in favor of imposing a tax on sugar-sweetened beverages. Awareness-raising campaigns and a policy of combining the measure with subsidies or tax cuts on healthy foods could increase the level of support among those currently against the intervention.