Royal jelly (RJ) contains 10-hydroxydecanoic acid (10HDAA), which is expected to have immune activation effects, such as promoting M cell differentiation. To enhance the use of RJ as a functional immunostimulatory food ingredient, it is desirable to increase its 10HDAA content. Our aim was to isolate bacteria capable of converting 10-hydroxy-2-decenoic acid (10H2DA) contained in RJ to 10HDAA, thereby applying the isolate for producing fermented RJ with a high 10HDAA content. A strain of lactic acid bacteria, strain M1, was successfully isolated from the digestive tract of queen bees, capable of converting 10H2DA to 10HDAA. It was identified as Lactobacillus panisapium. By cultivating strain M1 in a protease-treated RJ broth, fermented RJ (fRJ) containing five times the amount of 10HDAA compared to conventional RJ was produced. Preliminary evaluations of fRJ’s immune-stimulating effects revealed several benefits. It promoted the differentiation of M cells, which play a crucial role in intestinal immunity, activated the phagocytic ability of macrophages, and increased IgA secretion in individuals with reduced salivary immunoglobulin (Ig) A secretion. Safety tests confirmed that fRJ is safe for consumption. fRJ acts on M cells, which tend to decrease with age, and promotes salivary IgA secretion. This suggests that fRJ could be a novel immune-stimulating food rich in active ingredients.