This study was designed to assess the antimicrobial effect and tolerability of a single dose of a photo-activated gel containing RLP068 in the treatment for infected foot ulcers in subjects with diabetes. A randomized, double-blind, parallel series, placebo-controlled phase IIa trial was performed with three concentrations of RLP068 (0.10, 0.30, and 0.50 %), measuring total and pathogen microbial load on Day 1 (before and 1 h after topical gel application and photoactivation with 689 nm red light), on Days 3, 8, and 15, as add-on to systemic treatment with amoxicillin and clavulanic acid. Blood samples were also drawn 1, 2, and 48 h after administration for the assessment of systemic drug absorption. The trial was performed on 62 patients aged ≥18 years, with type 1 or type 2 diabetes and infected foot ulcer, with an area of 2-15 cm(2) and a maximum diameter ≤4.6 cm. A dose-dependent reduction in total microbial load was observed (-1.92 ± 1.21, -2.94 ± 1.60, and -3.00 ± 1.82 LogCFU/ml for 0.10, 0.30, and 0.50 % RPL068 vs. -1.00 ± 1.02 LogCFU/ml with placebo) immediately after illumination, with a progressive fading of the effect during follow-up. No safety issues emerged from the analysis of adverse events. Systemic absorption of RLP068 was negligible. Photodynamic antimicrobial treatment with RLP068 of infected diabetic foot ulcers is well tolerated and produces a significant reduction in germ load. Further clinical trials are needed to verify the efficacy of this approach as add-on to systemic antibiotic treatment.