Abstract We present an optical study of the strong-lensing galaxy cluster MS 0440.5+0204 at z = 0.19593, based on CFHT/MegaCam g′, r′ photometry and GMOS/Gemini and CFHT/MOS/SIS spectroscopy in a broader area than previous works. We have determined new spectroscopic redshifts for the most prominent gravitational arcs surrounding the central galaxy in the cluster. The new redshifts and the information provided by the photometric catalog allow us to perform a detailed weak- and strong-lensing mass reconstruction of the cluster. The large number of member galaxies and the area covered by our observations allow us to estimate more accurately the velocity dispersion and mass of the cluster and to examine in detail the nature of the cluster and surrounding structures. The dynamical mass is in good agreement with the mass inferred from the lensing analysis and X-ray estimates. About 68% of the galaxies are located in the inner ≲0.86 h 70 − 1 Mpc region of the cluster. The galaxy redshift distribution in the inner region of the cluster shows a complex structure with at least three substructures along the line of sight. Other substructures are also identified in the galaxy density map and in the weak-lensing mass map. The member galaxies in the northeast overdensity are distributed in a filament between the clusters MS 0440.5+0204 and ZwCL 0441.1+0211, suggesting that these two structures might be connected. MS 0440.5+0204 appears to be dynamically active, with a cluster core that is likely experiencing a merging process, and with other nearby groups at projected distances of ≲1 h 70 − 1 Mpc that could be being accreted by the cluster.