Significance A major component of the cell envelope of Gram-negative bacteria is LPS, also known as endotoxin. LPS produced during bacterial infections triggers inflammation, which can lead to septic shock and death. Our immune system can recognize LPS both outside and inside of cells. The recognition of extracellular and vacuolar LPS by LPS binding proteins is well described, but little is known about the recognition of cytoplasmic LPS. Here, we show that cytoplasmic LPS derived from the intracellular bacterial pathogen Legionella activated a proinflammatory immune response. We further identified host guanylate binding proteins as critical mediators of immunity triggered by cytoplasmic LPS. These findings are likely to advance our understanding of how cells can sense intracellular LPS.