Quinoa is a highly nutritious Andean crop with considerable yield potential that remains underexploited in southern Ecuador. This study evaluated the effects of planting method (row seeding, hill seeding, and transplanting) and plant density (8–20 plants m⁻²) on quinoa growth and yield under Andean highland conditions. A factorial field experiment was conducted using a randomized complete block design with three replicates. Plant density significantly affected grain yield, increasing from 4.4 to 4.8 t ha⁻¹ at 8 plants m⁻² to a maximum of 6.97 t ha⁻¹ at 20 plants m⁻². This increase was mainly driven by a higher grain number per unit area, while thousand-grain weight remained stable across treatments. In contrast, the planting method and its interaction with plant density had no significant effect on yield or yield components. Grain yield showed a strong positive relationship with above-ground biomass, indicating that biomass accumulation was the main driver of yield variation. These results demonstrate that plant density is the primary agronomic factor controlling quinoa productivity under Andean conditions. Optimizing plant density to 15–20 plants m⁻² is recommended as a simple and cost-effective management strategy to maximize grain yield, regardless of planting method.