Tetrapyrroles such as heme, chlorophyll, or vitamin B12are among the most complex molecules synthesised by nature and play a crucial role in many vital processes such as respiration, photosynthesis, methanogenesis, and catalysis. All modified tetrapyrroles are derived from the common precursor molecule 5-aminolevulinic acid (5-ALA). However, 5-ALA can be synthesised in two different ways. While alphaproteobacteria, mammals, and birds use the so-called Shemin pathway, all other bacteria, archaea, and plants use the C5 pathway. Here, we present evidence for the presence of 5-ALA synthase genes of the Shemin pathway in the genomes of viruses (valaS) that infect bacteria (bacteriophages) in marine and freshwater environments. These genes either occur in a three-gene cassette with two heme catabolising enzymes involved in linear tetrapyrrole biosynthesis, as broken cassettes or as individual genes. valaSencodes a functional enzyme, as shown by the enzymatic turnover of the co-substrates succinyl-Coenzyme A and glycine in anEscherichia colistrain overexpressing valaS. Phylogenetic analysis in combination with three-dimensional structure prediction supported our data that the viral sequences encode active enzymes. Interestingly, the viral valaSsequence is capable of functionally complementing a 5-ALA-auxotrophicEscherichia colistrain, which lacks one of the two enzymes of the C5 pathway. Host prediction suggests that different valaScarrying phages might infect bacteria that either utilise the Shemin or the C5 pathway for 5-ALA biosynthesis. Our results thus support the hypothesis that metabolic reprogramming and maintenance of the host bacterium’s energy metabolism are crucial during bacteriophage infection and that tetrapyrroles play a key role in this process.