The scarcity of animal feeding resources has been driving the use of sustainable alternatives such as Azolla. This study evaluated the effect of replacing concentrate feed mixture (CFM) with dried Azolla (DAZ) on the in vitro digestibility of rations, rumen fermentations, gas production, and rumen microbiota. The basal diet consisted of Berseem hay and CFM (50:50), and six rations were used, in which DAZ replaced the CFM at 0% (control), 10% (T1), 20% (T2), 30% (T3), 40% (T4), and 50% (T5). Group T1 showed higher degradability of dry matter (53.13%), organic matter (62.47%), neutral detergent fiber (30.79%), and acid detergent fiber (24.72%). The same group (T1) revealed the highest propionate and lowest methane production (p < 0.05). Principal coordinate analysis (PCoA) revealed that rumen microbial communities were affected by DAZ level. Microbial communities were dominated by the phylum Bacteroidota, which was higher in group T1, and the phylum Firmicutes, which was higher in group T2. The dominant bacterial genera were Prevotella, Rikenellaceae RC9 gut group, Streptococcus, and Christensenellaceae R-7 group, which were affected by DAZ level. Dried Azolla can be used up to 20% of CFM in ruminant rations without negative consequences on rumen fermentation.