Today, the global trend toward plant-based beverages has grown for sustainability, health-related, lifestyle, and dietary reasons. Among them, drinks produced from almonds have been recognized as a concentrated nutrient source. Commercial almond milk was fermented under the same processing conditions using water and milk kefir grains to determine the starter culture leading to the beverage with the better nutritional profile. The resulting fermented beverages were investigated for protein, phenolic, and flavonoid content, fatty acid profile, and antioxidant activity, determined by DPPH, ABTS, and FRAP assays. Comparing the results, it was found that the almond beverage from milk kefir grains had the highest protein. The lipid profile of both beverages was characterized by a high content of monounsaturated fatty acids and a lower saturated fatty acid concentration compared to almond milk. Despite the higher phenolic content of the almond beverage from milk kefir grains, the ABTS and DPPH tests showed increased antioxidant activity in both fermented beverages, but with no significant difference between them, while the FRAP test showed a pronounced predominance of iron-reducing ability in the beverage from water kefir grains. The evidence from this study suggested that both types of grains can be used as starter cultures to enhance the nutritional and bioactive properties of almond milk.