Resumo. Klebsiella pneumoniae is an opportunistic pathogen related to several cases of healthcare-associated and community-acquired infections worldwide, especially in Brazil. Numerous studies have shown that isolated secondary metabolites, such as phenolic acids, have the potential to act against this problem. This study aimed to investigate the inhibitory potential associated with phenolic acids on growth and biofilm formation in clinical isolates of Multidrug-Resistant and Extensively Drug-Resistant K. pneumoniae (MDR/XDR-KP). Four clinical isolates from a public hospital in Recife, Pernambuco, Brazil, and a sensitive standard strain were used. The initial identification of the samples was carried out using VITEK®2 and BD-PhoenixTM 100 automation equipment, as well as the characterization of the resistance profile. The samples were then confirmed using the MALDI-TOF/MS technique. The Crystal Violet method was used to assess biofilm formation capacity. Four phenolic acids (gallic, trans-ferulic, caffeic, and 4-hydroxybenzoic) were used to evaluate the antimicrobial and biofilm-forming activities. The isolates were confirmed as K. pneumoniae species with MALDI-TOF/MS scores ranging from 2.459-2.083. The samples showed both MDR and XDR resistance profiles, and biofilm formation with different intensities. Of all the compounds tested, caffeic and trans-ferulic acids were the most effective, with growth and biofilm inhibition values of 70-85% and 70-90% using a concentration of 2 mg/mL, respectively. Notably, K. pneumoniae belongs to a group considered by the WHO to be a critical public health priority to be combated. In this context, the results showed that phenolic acids had a great potential impact on both bacterial growth and the biofilm-forming capacity of MDR/XDR-KP clinical isolates. This leads us to recognize the use of phenolic acids as a possible alternative in the fight against infections caused by MDR, XDR, and biofilm-forming bacterial species.