The opportunistic pathogen Pseudomonas aeruginosa causes nosocomial infections affecting the urinary system, respiratory tract, etc. It can lead to bacteremia and sepsis, especially in immunocompromised patients. Growing antimicrobial resistance (AMR) is a global health concern according to the World Health Organization. Therefore, novel antimicrobial agents, such as plant extracts containing complexes of compounds to which it is impossible to develop resistance, are being investigated. This study aimed to compare the effects of ethyl acetate (EtOAc) extracts from the roots (EtOAcR) and aerial parts (EtOAcAP) of the perennial Bulgarian plant Geum urbanum L. (Rosacea) on the phenotype inhibition of the Las/RhI quorum sensing (QS) system in Pseudomonas aeruginosa PA01 and ATCC 27853, specifically on biofilm formation, swarming motility, pyocyanin production, and gene expression. For this purpose, we used sub-minimal inhibition concentrations (sub-MICs), which do not affect bacterial growth. We found that the evaluated sub-MICs suppressed all studied phenotypic manifestations, with no expression of the target lasI/lasR and rhII/rhIR genes. The observed anti-QS capacity of G. urbanum extracts is probably related to their high phenolic content. Moreover, the EtOAcR and EtOAcAP extracts showed effective antioxidant capacity via DPPH, ABTS, and superoxide radical scavenging effects, as well copper ion chelation activity. These findings will support the development of novel phytocomplexes applicable for the biocontrol and prevention of P. aeruginosa infections.