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The repertoire of modifications to bile acids and related steroidal lipids by host and microbial metabolism remains incompletely characterized. To address this knowledge gap, we created a reusable resource of tandem mass spectrometry (MS/MS) spectra by filtering 1.2 billion publicly available MS/MS spectra for bile-acid-selective ion patterns. Thousands of modifications are distributed throughout animal and human bodies as well as microbial cultures. We employed this MS/MS library to identify polyamine bile amidates, prevalent in carnivores. They are present in humans, and their levels alter with a diet change from a Mediterranean to a typical American diet. This work highlights the existence of many more bile acid modifications than previously recognized and the value of leveraging public large-scale untargeted metabolomics data to discover metabolites. The availability of a modification-centric bile acid MS/MS library will inform future studies investigating bile acid roles in health and disease. [Display omitted] • Mining of public, untargeted LC-MS/MS data reveals a large diversity of bile acids • This work created an MS/MS spectral library resource of modified bile acids • Polyamine biosynthetic pathway-derived metabolites amidated to bile acids discovered • American-to-Mediterranean diet transition altered levels of polyamine bile amidates Using mass spectrometry-based tools to mine public untargeted MS/MS data revealed the vastly underappreciated diversity of bile acids hidden in plain sight and led to the discovery of previously unknown polyamine-conjugated bile acids. [ABSTRACT FROM AUTHOR]