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<i>Phytophthora infestans</i>, the causal agent of late blight (LB) in tomato (<i>Solanum lycopersicum</i> L.), is a devastating disease and a serious concern for plant productivity. The presence of susceptibility (S) genes in plants facilitates pathogen proliferation; thus, disabling these genes may help provide a broad-spectrum and durable type of tolerance/resistance. Previous studies on <i>Arabidopsis</i> and tomato have highlighted that knock-out mutants of the <i>PMR4</i> susceptibility gene are tolerant to powdery mildew. Moreover, <i>PMR4</i> knock-down in potato has been shown to confer tolerance to LB. To verify the same effect in tomato in the present study, a CRISPR–Cas9 vector containing four single guide RNAs (sgRNAs: sgRNA1, sgRNA6, sgRNA7, and sgRNA8), targeting as many <i>SlPMR4</i> regions, was introduced via <i>Agrobacterium</i>-<i>tumefaciens</i>-mediated transformation into two widely grown Italian tomato cultivars: ‘San Marzano’ (SM) and ‘Oxheart’ (OX). Thirty-five plants (twenty-six SM and nine OX) were selected and screened to identify the CRISPR/Cas9-induced mutations. The different sgRNAs caused mutation frequencies ranging from 22.1 to 100% and alternatively precise insertions (sgRNA6) or deletions (sgRNA7, sgRNA1, and sgRNA8). Notably, sgRNA7 induced in seven SM genotypes a −7 bp deletion in the homozygous status, whereas sgRNA8 led to the production of fifteen SM genotypes with a biallelic mutation (−7 bp and −2 bp). Selected edited lines were inoculated with <i>P. infestans</i>, and four of them, fully knocked out at the <i>PMR4</i> locus, showed reduced disease symptoms (reduction in susceptibility from 55 to 80%) compared to control plants. The four SM lines were sequenced using Illumina whole-genome sequencing for deeper characterization without exhibiting any evidence of mutations in the candidate off-target regions. Our results showed, for the first time, a reduced susceptibility to <i>Phytophtora infestans</i> in pmr4 tomato mutants confirming the role of KO <i>PMR4</i> in providing broad-spectrum protection against pathogens.