Genomic analyses in budding yeast have helped to define the foundational principles of eukaryotic gene expression, but have systematically excluded specific classes of potential coding regions, including those with non-AUG start codons. Without methods to define coding regions empirically, the prevalence of these non-canonical coding regions has been impossible to assess. Here, we applied an experimental approach to globally annotate translation initiation sites in yeast and identified a class of 149 genes that encode N-terminally extended alternate protein isoforms that result from translation initiation at non-AUG codons upstream of the annotated AUG start codon. These alternate isoforms are produced in concert with canonical isoforms and are translated with a high degree of specificity, resulting from initiation at only a small subset of possible start codons in 5’ leader regions. Their translation is enriched during meiosis, and is induced by low eIF5A levels, which are observed in this context. These findings reveal widespread production of non-canonical protein isoforms and, more generally, show unexpected complexity to the rules by which the budding yeast genome is decoded. Overall design: We performed ribosome profiling following pre-treatment of budding yeast cells with 3 micromolar of the translation initiation inhibitor, lactimidomycin (LTM).