Patients with autosomal dominant polycystic kidney disease (ADPKD), a genetic disease due to mutations of the PKD1 or PKD2 gene, show signs of complement activation in the urine and cystic fluid, but their pathogenic role in cystogenesis is unclear. We tested the causal relationship between complement activation and cyst growth using a Pkd1 ^KO renal tubular cell line and newly generated conditional Pkd1 ^–/– C3 ^–/– mice. Pkd1 -deficient tubular cells have increased expression of complement-related genes ( C3 , C5 , CfB , C3ar , and C5ar1 ), while the gene and protein expression of complement regulators DAF, CD59, and Crry is decreased. Pkd1 ^–/– C3 ^–/– mice are unable to fully activate the complement cascade and are characterized by a significantly slower kidney cystogenesis, preserved renal function, and reduced intrarenal inflammation compared with Pkd1 ^–/– C3 ^+/+ controls. Transgenic expression of the cytoplasmic C-terminal tail of Pkd1 in Pkd1 ^KO cells lowered C5ar1 expression, restored Daf levels, and reduced cell proliferation. Consistently, both DAF overexpression and pharmacological inhibition of C5aR1 (but not C3aR) reduced Pkd1 ^KO cell proliferation. In conclusion, the loss of Pkd1 promotes unleashed activation of locally produced complement by downregulating DAF expression in renal tubular cells. Increased C5a formation and C5aR1 activation in tubular cells promotes cyst growth, offering a new therapeutic target.