Infiltration of plasma cells can be a histopathological hallmark of articular synovium with rheumatoid arthritis (RA). A proliferation-inducing ligand (APRIL) may have key roles in homeostasis and development of B cells, and the differentiation of B cells into plasma cells. This study was designed to explore the relationships between the infiltrations of plasma cells in synovium and the synovial fluid levels of APRIL in inflamed peripheral joints of RA. Synovium and synovial fluid were sampled from 21 RA patients underwent arthroscopic synovectomy for inflamed peripheral joints. The variants of rheumatoid synovium were classified into the follicular and diffuse synovitis by hematoxylin and eosin staining, and the infiltrations of plasma cells in rheumatoid synovium were quantified under the light microscope. The synovial fluid levels of APRIL were measured with the enzyme-linked immunosorbent assay. The mean number of infiltrating plasma cells in synovium and the mean synovial fluid level of APRIL were significantly increased in follicular synovitis compared with those in diffuse synovitis (P = 0.009, and P = 0.018, respectively), and there was a highly positive association between the infiltrations of plasma cells and the synovial fluid levels of APRIL among all of the RA patients (Rs = 0.776, P < 0.001). These findings suggest that the local production of APRIL may be associated with the ectopic lymphoid neogenesis in rheumatoid synovium and may have a role in contributing to the infiltration of plasma cells in synovium within inflamed peripheral joints of RA.