The response of watermelon variety ‘sugar baby’ to three levels of phosphorous (0,50 and100 Kg P2O5/ha) was tested in a field experiments in KARI, Matuga, Kwale District in the coastal region of Kenya. A randomized complete block design with three replications was used. Phosphorous had a positive significant effect on the sex expression ratio, number of fruits/plant, fruit weights, firmness, rind thickness, total soluble solids. Application of 50 and 100 Kg P2O5/ha increased the fruit sugar content by 10% and 4% compared with zero application. The total fruit yield was highest at 50 and 100 Kg P2O5/ha,with increasing fruit yield of 16% and16.4% respectively compared with zero application. Rind thickness, fruit firmness decreased and increased with increasing levels of P respectively. The watermelon sex ratio significantly increased with increase in phosphorous up to a point (more than 50 Kg P2O5/ha) when further increase led to a reduction signifying an increase in female flowers. For improved growth, yield, total soluble solids and firmness of water melons in the coastal region of Kenya 50 Kg P2O5/ha was recommended.