Artocarpus tonkinenesis (Moraceae) has been used in Vietnamese traditional medicine for the treatment of backache and joint diseases since many 100 years. We have previously shown that a crude extract of A. tonkinensis elicited anti-inflammatory effects in rat collagen-induced arthritis (CIA), with significant improvement of disease symptoms. However, the pharmacological basis of the bioactivity of A. tonkinensis extract is not known. In the present study, we have isolated four individual active components from A. tonkinensis extract by reverse phase high-pressure liquid chromatography. The structures of the compounds were determined by nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry and their biological effects investigated. A novel biologically active flavonoid glucoside (5-hydroxy-8-hydroxymethyl-8-methyl-2-[4-(3,4,5-trihydroxy-6-hydroxymethyl-tetrahydro-pyran-2-yloxy)-phenyl]-8H-pyrano[3,2-g]chromen-4-one) with an average molecular mass of 514.49 Da was isolated. We have named the compound artonkin-4′-O-glucoside. The name ‘artonkin’ for the novel flavonoid part of the compound was coined from the Latin name of its source Artocarpus tonkinensis. The three other active flavonoid glucosides isolated and characterized were alphitonin-4-O-β-d-glucoside, maesopsin-4-O-β-d-glucoside and kaempherol-3-O-β-d-glucoside. All four compounds were found to cause anti-inflammatory effect with different potencies. The anti-inflammatory effects demonstrated in the rat model of arthritis correlate well with the inhibition of mitogen-induced T-cell proliferation. Furthermore, the compounds inhibit production of cytokines, such as tumour necrosis factor-α and interferon-γ, in mitogen-stimulated T cells in a concentration-dependent manner. We postulate that the isolated flavonoids suppress T-cell proliferation as well as cytokine expression and thereby contribute to an amelioration of arthritis severity in CIA.