Recently, various deep learning-based methods have been designed to improve the spectral resolution of the multispectral image (MSI) to obtain the hyperspectral image (HSI). These methods usually rely on sufficient MSI/HSI pairs for supervised training. However, collecting plentiful HSIs is time-consuming. In this letter, a semisupervised spectral degradation constrained network (SSDCN) is proposed to improve the spectral resolution of MSI. SSDCN is an autoencoder-like network that is composed of an encoder subnetwork for estimating HSI from input MSI and a decoder subnetwork for reconstructing MSI from the estimated HSI. A semisupervised training method is proposed to explore both MSI/HSI pairs and MSIs without ground-truth HSIs to optimize SSDCN. Simulated and two real databases are employed to demonstrate the effectiveness of SSDCN. IEEE