Tamarix ramosissima is a traditional Chinese herbal medicine used for rheumatoid arthritis (RA) treatment in Northwest China. Chemical investigation of EtOH/H2O extracts of T. ramosissima led to the discovery of a new flavonol, ramosissimin (1), together with the known flavonoids compounds quercetin (2), quercetin-3'4'-dimethylether (3) and kaempferol (4). By means of high resolution electrospray ionization mass spectroscopy (HRESIMS) and 1D and 2D-NMR experiments, and after comparison with literature data, the structures of the compounds were determined. The effect of compound 1 on the viability of RA fibroblast-like synoviocytes (RA-FLS) was evaluated by MTT assay. Apoptosis-inducing effect of compound 1 in RA-FLS was further investigated by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay and activated caspase-3/7 level assessment using luminescent assay. The results revealed that ramosissimin displayed remarkable proliferation inhibitory effect in RA-FLS. Furthermore, compound 1 could significantly induce cellular apoptosis of RA-FLS and increase activated caspase-3/7 levels. It is suggested that ramosissimin may inhibit the proliferation of RA-FLS by inducing apoptosis.