This study investigated the effects of incremental dietary levels of marula seed cake (MSC) in partial replacement of soyabean meal (SBM) and maize on growth performance, viscera macromorphometry, carcass traits, and haemato-biochemistry of broiler chickens during the starter, grower, and finisher phases. In a completely randomized design, 400 day-old Ross 308 broilers were randomly allocated to five diets with 0, 5, 10, 15, and 20% MSC, each with eight replicates of 10 (five of each sex). Weekly feed intake (FI), body weight gain (BWG), and feed conversion efficiency (FCE) were calculated; haemato-biochemistry was measured at day 42. FI was quadratically decreased by dietary MSC, of which the optimum inclusion was 150 g/kg, as BWG and FCE were linearly decreased by the marula by-product. MSC linearly decreased bird slaughter weight and hot and cold carcass weight. White blood cells, lymphocytes, and symmetric dimethylarginine decreased linearly but serum cholesterol concentrations increased linearly. Dietary inclusion of MSC at levels >150 g/kg induced detrimental effects on productive performance, visceral organs and development, carcass traits, and haemato-biochemistry of broiler chickens. There is therefore a need for strategies to resolve antinutritional effects of high dietary MSC to optimize its inclusion in broiler diets.