To define the cell populations that drive joint inflammation in rheumatoid arthritis (RA), we applied single-cell RNA sequencing (scRNA-seq), mass cytometry, bulk RNA sequencing (RNA-seq) and flow cytometry to Tcells, B cells, monocytes, and fibroblasts from 51 samples of synovial tissue from patients with RA or osteoarthritis (OA). Utilizing an integrated strategy based on canonical correlation analysis of 5,265 scRNA-seq profiles, we identified 18 unique cell populations. Combining mass cytometry and transcriptomics revealed cell states expanded in RA synovia: THY1(CD90)(+)HLA-DRA(hi) sublining fibroblasts, IL1B(+) pro-inflammatory monocytes, ITGAX(+)TBX21(+) autoimmune-associated B cells and PDCD1(+) peripheral helper T (T-PH) cells and follicular helper T (T-FH) cells. We defined distinct subsets of CD8(+)Tcells characterized by GZMIK(+), GZMB(+), and GNLY(+) phenotypes. We mapped inflammatory mediators to their source cell populations; for example, we attributed 1L6 expression to THYl(+)HLA-DRA(hi) fibroblasts and IL1B production to pro-inflammatory monocytes. These populations are potentially key mediators of RA pathogenesis.