Background/Objectives: Arnica montana L. is widely recognized for its diverse biological activities, including antimicrobial effects. This study aimed to evaluate the antimicrobial and antibiofilm activity of A. montana L. extracts against Acinetobacter baumannii, a pathogen of urgent public health concern due to its increasing antibiotic resistance and capacity for biofilm formation. Methods: The antimicrobial activity of ethanolic (EtE) and aqueous (AqE) extracts of A. montana flowers was evaluated via the broth microdilution method. The minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC), and the MBC/MIC ratio were used. The effects of EtE on A. baumannii biofilm formation were assessed via a crystal violet assay. Additionally, transcriptional profiling of biofilm-associated genes following exposure to sub-MIC levels of the extract was conducted via RT-qPCR. Results: The anti-Acinetobacter activity of EtE was demonstrated (MIC = 234.4 and 468.75 µg/mL for A. baumannii ATCC BAA-3252 and ATCC 19606, respectively). The EtE exhibited bactericidal activity against both strains, whereas the AqE showed no activity. Additionally, EtE inhibited biofilm formation and significantly downregulated the expression of key biofilm-associated genes, including those of the csu operon and ompA. Conclusions: Arnica montana EtE demonstrated antimicrobial and antibiofilm activities against A. baumannii and inhibited biofilm development by suppressing the transcription of genes involved in pilus assembly and surface adherence, highlighting their essential role in biofilm formation.