In this article we describe and evaluate a public participation geographic information systems (PPGIS) methodology for use in national forest planning in the United States. The Internet-based mapping method collects public landscape values and special places data for input into a national forest planning decision support system. The PPGIS method presupposes that, within their existing statutory framework, national forests should be managed for compatibility with the range of values the public holds for those lands and that inadequate understanding, modeling, and documentation of place-specific forest values can negatively affect public acceptance, viability, and the usefulness of national forest plans. In this article we 1) review previous applications of landscape value mapping methods across a variety of planning applications, 2) describe the participatory, Internet mapping method used in three studies of national forests in Arizona and Oregon in 2006 and 2007, 3) present and evaluate the results to show likely future implementation constraints, and 4) based on lessons learned, describe a recommended PPGIS protocol for national forest planning. The results from the three studies demonstrate that an Internet, participatory mapping method, although not without limitations, can be effective in measuring landscape value and special place data for use in a variety of forest planning processes. Because the protocol expands and in a sense democratizes the agency's public participation process, implementation of the PPGIS protocol across the national forest system can help restore public trust in the agency's forest planning processes and forest management decisions.