There is increasing interest in altitude effects on structural attributes and aboveground biomass (AGB) in tropical forests. However, we continue to lack a clear understanding of relationships between structural attributes and AGB along altitudinal gradients in Central Africa. Using a new network of 76 permanent plots of 0.5 ha, the relationships between structural attributes and AGB were explored along fine-scale altitudinal gradients in the Republic of Congo. We chose four fine-scale altitudinal gradients (71–350 m, 350–550 m, 550–650 m, and 650–853 m) and measured the diameter and heights of 4192 trees with a ≥10 cm diameter and calculated the structural attributes and the AGB for each 0.5 ha plot. For a given diameter, trees were shorter and had narrower crowns in the altitudinal gradient of 71–350 m than in the other altitudinal gradients. The relationships between structural attributes and AGB differ along fine-scale altitudinal gradients, with higher stem density, wood density, crown depth, and AGB for the altitudinal gradient of 71–350 m. These results provide important advances in our understanding of the relationships between structural attributes and AGB along fine-scale altitudinal gradients in Central Africa. They should improve AGB estimates for the low altitudinal gradient when combined with other field and remotely sensed data sets in Central Africa.