The use of synergistic combinations between natural compounds and commercial antibiotics may be a good strategy to fight against microbial resistance, with fewer side effects on human, animal and environmental, health. The antimicrobial capacity of four compounds of plant origin (thymol and gallic, salicylic and gentisic acids) was analysed against 14 pathogenic bacteria. Thymol showed the best antimicrobial activity, with MICs ranging from 125 µg/mL (forAcinetobacter baumannii,Pasteurella aerogenes,andSalmonella typhimurium) to 250 µg/mL (forBacillus subtilis,Klebsiella aerogenes,Klebsiella pneumoniae,Serratia marcescens,Staphylococcus aureus,andStreptococcus agalactiae). Combinations of thymol with eight widely used antibiotics were studied to identify combinations with synergistic effects. Thymol showed synergistic activity with chloramphenicol againstA. baumannii(critical priority by the WHO), with streptomycin and gentamicin againstStaphylococcus aureus(high priority by the WHO), and with streptomycin againstStreptococcus agalactiae, decreasing the MICs of these antibiotics by 75% to 87.5%. The kinetics of these synergies indicated that thymol alone at the synergy concentration had almost no effect on the maximum achievable population density and very little effect on the growth rate. However, in combination with antibiotics at the same concentration, it completely inhibited growth, confirming its role in facilitating the action of the antibiotic. The time–kill curves indicated that all the combinations with synergistic effects were mainly bactericidal.