A water-soluble polysaccharide (JCS1) was isolated from the stems of Dendrobium nobile Lindl. JCS1 was structurally characterized using a combination of chemical and spectral analysis, including methylation analysis, partial acid hydrolysis, Fourier-transform infrared (FTIR) spectroscopy, gas chromatography (GC), GC-mass spectrometry (MS), and nuclear magnetic resonance (NMR) spectroscopy. The molecular weight was estimated to be 2.3 × 10⁴ Da using high-performance gel permeation chromatography (HPGPC). The sugar composition analysis indicated it was composed of glucose, mannose, xylose, and arabinose in a 40.2:2.3:1.7:1.0 molar ratio. The structure analysis showed that JCS1 was a mannoglucan with a backbone consisting of (1→4)-linked β-Manp and (1→4)-linked α-Glcp with branches at C-6 of (1→4)-linked α-Glcp residues. The branches were composed of T-α-Glcp, 1,4-α-Xylp, and T-α-Araf. In vitro bioactivity tests revealed that the acetylated derivative of JCS1, YJCS1, induced neuritogenesis of PC-12 cells. These results demonstrate that YJCS1 might be a promising bioactive polysaccharide for development as a drug candidate for the possible prevention and treatment of neurodegeneration diseases.