Objective: The purpose was to evaluate the yield, the chemical composition and the in vitro digestibility of Guinea grass (Megathyrsus maximus, Jacq.) cv. Mombasa according to plant spacing and season of the year. Design/methodology/approach: The Mombasa grass was stablished at spacing between plants of: 25x25, 50x25, 70x25 and 100x25 cm between rows and plants, respectively, and with broadcasting (traditional planting); cutting was conducted in cool, dry and rainy seasons. A factorial split-plot design in randomized blocks was used, with four replications. Results: The dry matter yield (DMY) of Mombasa grass was equal (p˃0.05) in cool and rainy seasons and lower (p<0.05) in dry season (27.2, 27.5 and 12.7 kg ha-1 d-1, respectively). The crude protein (CP) increased with greater plant spacing (p<0.05). The fodder of the dry season showed a higher (p<0.05) CP content than cool or rainy seasons (10.5, 9.7 and 8.7%). The content of all fiber fractions, except lignin, did not differ between plant spacing (p>0.05). Study limitations/implications: The season of the year affects the quality of tropical grasses. On the other hand, there is no information about the effect of plant spacing on the quality of Mombasa grass. Findings/conclusions: The highest DMY was in the cool season at 75x25 cm and 100x25 cm. The fodder harvested in the dry season showed lower fiber content in cell walls and higher in vitro digestibility (p˂0.05), but lower DMY. To ensure proper yield and quality, it is recommended to establish the Guinea grass cv. Mombasa at greater plant spacing, such as 75x25 cm or 100x25 cm.