Previously we have identified a conserved domain (SAP, for SAF-A/B, Acinus, and PIAS) in the foot-and-mouth disease virus (FMDV) leader (L) protein coding region that is required for proper sub-cellular localization and function. Mutation of isoleucine 55 and leucine 58 to alanine (I55A, L58A) within the SAP domain resulted in a viable virus that displayed a mild attenuated phenotype in cell culture, along with altered sub-cellular distribution of L and failure to induce degradation of the transcription factor nuclear factor kappa-B. Here we report that inoculation of swine and cattle with this mutant virus results in the absence of clinical disease, the induction of a significant FMDV-specific neutralizing antibody response, and protection against subsequent homologous virus challenge. Remarkably, swine vaccinated with SAP mutant virus are protected against wild type virus challenge as early as two days post-vaccination suggesting that a strong innate as well as adaptive immunity is elicited. This variant could serve as the basis for construction of a live-attenuated FMD vaccine candidate.