Understanding core and conditional gene essentiality is crucial for decoding genotype-phenotype relationships in organisms. We present phage high-throughput approach for gene essentiality mapping and profiling (PhageMaP), a high-throughput method to create genome-scale loss-of-function libraries for systematically assessing gene essentiality in bacteriophages. Using PhageMaP across diverse hosts, we generate gene essentiality maps for all or most genes in the model phages T7 and T4, as well as the non-model phage Bas63. These maps provide fundamental insights into genome organization, gene function, and host-specific conditional essentiality. By applying PhageMaP to a collection of anti-phage defense systems, we uncover phage genes that either inhibit or activate six defenses and offer mechanistic hypotheses. Furthermore, we engineer synthetic phages with enhanced infectivity by modular transfer of a PhageMaP-discovered defense inhibitor from Bas63 to T7. PhageMaP advances bacteriophage functional genomics and accelerates rational phage design for therapy.