Correspondence:We introduce a web-enabled small-molecule mass spectrometry (MS) search engine. To date, no tool can query all the public small-molecule tandem MS data in metabolomics repositories, greatly limiting the utility of these resources in clinical, environmental and natural product applications. Therefore, we introduce a Mass Spectrometry Search Tool (MASST) (https://proteosafe-extensions.ucsd.edu/masst/), that enables the discovery of molecular relationships among accessible public metabolomics and natural product tandem mass spectrometry data (MS/MS).The ability to discover related sequences of proteins or genes in publicly accessible sequence data using Basic Local Alignment Search Tool (BLAST), connected to public sequence data repositories through a web interface (WebBLAST, https://blast.ncbi.nlm.nih.gov/Blast.cgi), was introduced in the 1990s. 1 It has garnered more than 138,159 citations according to Google Scholar, placing it among the most widely used bioinformatics tools. WebBLAST enabled detection of the number of sequences in public repositories related to a given query, the organisms in which those sequences occur, and the evolutionary and inferred functional relationships among related sequences. It therefore permitted a broad community to answer simple but scientifically compelling questions such as: Is a protein or DNA sequence common or rare? How is this sequence distributed among different kinds of organisms? What other sequences are related to this sequence (evolutionary variants, or new mutations, or synthetic constructs)? In the early days of making DNA or protein sequence data publicly available, the "metadata" (e.g., contextual information about the sample, population and location the sequence came from, and technical information about how it was produced) in the public repositories was limited and no standards existed. This is a situation similar to the current status of much of the mass spectrometry data in the public domain. However, when publicly deposited data has metadata available, such as organism, location of sampling, host phenotypes such as diseases, etc., it becomes possible to start building higherlevel hypotheses regarding the evolutionary, ecological or functional relationships among these DNA, RNA or protein sequences. The development of the ability to search data with added context continues to have profound impacts on fields including medicine, chemistry, genetics, molecular biology, genomics, microbiology, and ecology.Algorithms developed for mass spectrometry data, including molecular networking 2 and fragmentation trees 3 , enable similarity searches, while powerful metabolomics analysis software infrastructures, such as MS-DIAL 4 , MetaboAnalyst 5 , XCMS Online 6 , HMDB 7 , some of which have been available for over a decade, focus on annotation of MS/MS spectra or finding statistical relationships between molecular features. However, none of the existing tools enable searching against public data in repositories. Finding the distribution of specific data of i...