<p>Poultry slaughterhouse wastewater (PSW) is a source of environmental pollutants, harboring pathogens and antibiotic resistance genes (ARGs). This study aimed to assess the effects of conventional biological treatment of PSW on the bacterial community and its efficiency in removing ARGs, as well as to evaluate the impact of its discharge on the receiving river. Samples were collected from raw sewage, treated effluent, and upstream and downstream river sites. Total metagenomic DNA was extracted for real-time PCR quantification of 16S rRNA, <i>yccT</i> gene (<i>Escherichia coli</i>), and ARGs, which were selected based on their ability to confer resistance to clinically relevant antibiotics and their prevalence in poultry-associated environments, including resistance to tetracyclines (<i>tetM</i>), beta-lactams (<i>bla</i>_<i>TEM</i>), sulfonamides (<i>sul1</i>), and quinolones (<i>qnrS</i>). Amplicon sequencing of 16S rRNA V3–V4 region was used to assess bacterial community structure. Treated effluent significantly altered the downstream microbiome, reducing bacterial richness by up to 72.3% and diversity by 25.4%. Effluent-associated phyla such as Pseudomonadota (37%), Bacillota (28%), and Bacteroidota (26%) became dominant in the downstream river samples. Enterobacterales increased after treatment, and <i>E. coli</i> increased by 2.93 logs downstream. All ARGs increased after treatment and remained elevated downstream, with <i>qnrS</i> and <i>sul1</i> rising by 3.77 and 3.87 logs, respectively. These findings highlight PSW treatment plants as a potential point of selection and dissemination of antimicrobial resistance (AMR)-related bacteria and genes. Inefficient treatment contributes to shifts in river bacterial communities and the spread of AMR.</p>