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Simple Summary: Multiple myeloma (MM) is characterized by the presence of systemic clinical manifestations. Among them, pneumonia accounts for a significant cause of morbidity and mortality, highlighting an urgent need to explore possible susceptibility and potential mechanisms of pneumonia in MM. Recently, the gut–lung axis has emphasized the relevance of gut microbiota and pneumonia, indicating that the compromised function of gut microbiota may be susceptible to pneumonia. It has been previously shown that intestinal Klebsiella pneumonia enriches in MM patients and promotes MM progression in our previous work. In addition, host metabolism has been identified as a key regulator of MM. However, what role the altered gut microbiota plays in MM with pneumonia remains unknown. In this study, we explored the association between gut microbiota and MM with pneumonia. This is the first study to identify and elucidate the function of gut microbiota involved in MM with pneumonia. Pneumonia accounts for a significant cause of morbidity and mortality in multiple myeloma (MM) patients. It has been previously shown that intestinal Klebsiella pneumonia (K. pneumonia) enriches in MM and promotes MM progression. However, what role the altered gut microbiota plays in MM with pneumonia remains unknown. Here, we show that intestinal K. pneumonia is significantly enriched in MM with pneumonia. This enriched intestinal K. pneumonia links to the incidence of pneumonia in MM, and intestinal colonization of K. pneumonia contributes to pneumonia in a 5TGM1 MM mice model. Further targeted metabolomic assays reveal the elevated level of glutamine, which is consistently increased with the enrichment of K. pneumonia in MM mice and patients, is synthesized by K. pneumonia, and leads to the elevated secretion of TNF-α in the lung normal fibroblast cells for the higher incidence of pneumonia. Inhibiting glutamine synthesis by establishing glnA-mutated K. pneumonia alleviates the incidence of pneumonia in the 5TGM1 MM mice model. Overall, our work proposes that intestinal K. pneumonia indirectly contributes to pneumonia in MM by synthesizing glutamine. Altogether, we unveil a gut–lung axis in MM with pneumonia and establish a novel mechanism and a possible intervention strategy for MM with pneumonia. [ABSTRACT FROM AUTHOR]