Mechanisms of crosstalk between tumor cells and tumor-associated macrophages (TAM) which drive metastasis are not fully understood. Scavenger receptor A1 (SR-A1) expressed primarily in macrophages has been associated with lung tumorigenesis. In this study, we used population genetics, transcriptomics and functional analyses to uncover how SR-A1 is involved in lung cancer and its prognosis. SR-A1 genetic variants were investigated for possible association with survival of advanced-stage NSCLC patients in the Harvard Lung Cancer Study cohort. Two SNPs (rs17484273, rs1484751) in SR-A1 were associated significantly with poor overall survival in this cohort. Data from The Cancer Genome Atlas showed considerable downregulation of SR-A1 in lung tumor tissues. The association of SR-A1 with prognosis was validated in animal models in the context of lung cancer metastasis. Macrophages derived from mice genetically deficient for SR-A1 exhibited accelerated metastasis in a model of lung cancer. On the other hand, tumor cell seeding, migration and invasion as well as macrophage accumulation in lung cancer tissue were enhanced in SR-A1 deficient mice. SR-A1 deletion upregulated serum amyloid A1 (SAA1) in macrophages via MAPK/Iκ-B/NF-κB signaling. SAA1 promoted tumor cell invasion and macrophage migration in vitro and in vivo, but these effects were blocked by administration of an anti-SAA1 antibody. Overall, our findings show how SR-A1 suppresses lung cancer metastasis by downregulating SAA1 production in tumor-associated macrophages.