Introduction: Limited rainfall in semi-arid regions can lead to forage deficiencies necessitating protein supplementation for beef cattle diets. The objective of this study was to investigate the effects of protein supplement type and delivery method on rumen bacterial communities in beef cattle. We hypothesized that rumen degradable protein (RDP) supplement would enrich for proteolytic and fiber-degrading microorganisms, and the effects of protein supplementation would be modulated by the delivery method due to differences in intake rate, which would lead to greater ammonia accumulation in self-fed animals. Methods: Treatments were RDP or rumen undegradable protein (RUP) delivered via hand-feeding (HF) or selffeeding (SF). Rumen samples were collected at 0 (pre) and 8 h post-feeding for 16S rRNA gene sequencing for bacterial community analysis. Results and discussion: No differences in alpha diversity were observed between protein supplements, but hand-fed animals had 40% greater alpha diversity when fed the RDP supplement compared to SF, and a protein & times; delivery method interaction (p = 0.02) was also observed. The genus Blautia had 2-fold greater relative abundance (p < 0.05) in RUP-supplemented animals and was positively correlated with butyric acid concentration (Pearson's r = 0.6, p < 0.01), while Streptococcus lutetiensis had 2-fold greater relative abundance (p < 0.05) in RDP-supplemented animals. Co-occurrence network analysis revealed greater connectivity in RDP-supplemented communities, with Prevotellaceae comprising 33% of keystone taxa compared with a single representative in RUP communities. Results demonstrated an interaction between supplement type and delivery method, indicating that how supplements are provided (controlled vs. ad libitum) can significantly alter microbial outcomes. These results have important implications for beef cattle production in semi-arid regions where forage quality is often limiting. Producers should consider RDP supplementation during periods of poor forage quality to optimize rumen microbiota function and enhance forage utilization efficiency. However, the delivery method must be carefully managed, as unrestricted access to RDP supplements can reduce bacterial community diversity.