Rosmarinic acid (RA) is an attractive candidate for ulcerative colitis (UC) application due to its bioactive properties, including antioxidant and anti-inflammatory functions, however, the poor water solubility and on-targeting hamper its therapeutic outcome. Therefore, this work reported the synthesis and preparation of novel water-soluble rosmarinic acid-chondroitin sulfate A (RA-CSA) nanoconjugate, which was used for the treatment of UC in dextran sulfate sodium (DSS)-induced acute colitis mouse model. RA was functionalized with CSA as confirmed by FTIR and (1)H NMR, and self-assembled to form nanoassemblies with a diameter of 247.3 +/- 2.99 nm. RA-CSA nanoassemblies exhibited radical scavenging and antioxidant capacity. RA-CSA remarkably inhibited lipopolysaccharide-induced nitric oxide and TNF-alpha production in RAW 264.7 cells without cytotoxicity, whose inhibition rate was <5 % at 200 mug mL(-1). Oral administration of RA-CSA nanoassemblies significantly attenuated colonic inflammation compared to the parent RA, as evidenced by significantly reduced the shortening of colon length (4.20 +/- 0.15 cm), body weight loss, and colonic inflammatory damage in DSS-induced colitis mice. In addition, RA-CSA nanoassemblies suppressed the expression and production of typical pro-inflammatory cytokines of ulcerative colitis. These results suggest that RA-CSA nanoassemblies deserve further consideration as a potential therapeutic drug for the treatment of UC.