The U.S. Water Resources Council has considered objectives in addition to national economic development for the evaluation of water projects. Cost‐sharing rules have not been proposed for the new objectives. A conceptual method is proposed for planning the nationally optimal project with multiobjectives. The data required for project evaluation with multiobjectives are used to apply a cost‐sharing rule that induces local interests to choose nationally efficient projects. The rule is recommended for traditional project purposes and some new purposes. It can be applied to a multiple‐purpose project without allocating project joint costs. A ceiling and a floor on local percentage cost shares of 100% and 20% of project costs, respectively, are considered for traditional purposes. Local percentage cost shares of 50% and 100% are considered for all costs allocated to environmental quality and regional development objectives, respect