The benefits of Lagos spinach (Celosia argentea L.) as a medicinal plant and leafy vegetable encourage its production. However, goat weed (Ageratum conyzoides L.) is a common weed in the agroecological region where C. argentea thrives. Evaluation of the goat weed effect on C. argentea is necessary since the impact of crop-weed interaction varies with species and density. A screen-house study comprising a C. argentea plant with 0, 2, 4, 6, 8, and 10 goat weed plants per pot were laid out in a completely randomized design and replicated six times. The experimental treatments were equivalent to 0, 100, 200, 300, 400, and 500 goat weed plants per square meter. Growth parameters of C. argentea, such as plant height, number of leaves and number of branches, were recorded weekly. The study also analyzed weight, moisture, ash, lipid, dietary fiber, protein, and carbohydrate content of C. argentea after harvest. The results showed that all the goat weed densities negatively impacted the growth of C. argentea. However, 8 and 10 goat weed plants per pot seemed to have the greatest effect on the growth of C. argentea. The moisture content, ash, crude protein, and crude fiber of C. argentea were significantly reduced by 50–60%, 60–69%, 45–56%, and 42–54%, respectively, due to the goat weed densities, whereas the carbohydrate content increased. Hence, goat weed should be maintained at less than 100 plants per square meter to prevent quantitative and qualitative losses.