IgBLAST is a sequence analysis tool designed for immunoglobulin (IG) variable domain analysis. It addresses the limitations of general-purpose tools like BLAST in handling IG sequences, which are rearranged and have variable gene lengths. IgBLAST can analyze both nucleotide and protein sequences, process batches of sequences, and search against germline gene databases and other sequence databases simultaneously to minimize the risk of missing the best matching germline V gene. The tool provides detailed information on gene matches, rearrangement junctions, and FR/CDR delineation. It uses the BLAST algorithm and is implemented using the NCBI C++ toolkit. IgBLAST identifies V, D, and J gene hits through multiple BLAST searches, with adjustable parameters for D gene detection. It also determines the rearrangement frame and provides a clear summary of search results, including alignment details and match statistics. The tool is robust and can handle various query formats, including FASTA, GenBank, and GI numbers. It is a free public tool with no login requirement. Evaluation of IgBLAST against other tools like iHMMune-align showed that it performs well in identifying germline genes and rearrangements, particularly in clonally related sequences. IgBLAST is a valuable resource for IG sequence analysis, offering a comprehensive and flexible solution for researchers in immunology and bioinformatics.IgBLAST is a sequence analysis tool designed for immunoglobulin (IG) variable domain analysis. It addresses the limitations of general-purpose tools like BLAST in handling IG sequences, which are rearranged and have variable gene lengths. IgBLAST can analyze both nucleotide and protein sequences, process batches of sequences, and search against germline gene databases and other sequence databases simultaneously to minimize the risk of missing the best matching germline V gene. The tool provides detailed information on gene matches, rearrangement junctions, and FR/CDR delineation. It uses the BLAST algorithm and is implemented using the NCBI C++ toolkit. IgBLAST identifies V, D, and J gene hits through multiple BLAST searches, with adjustable parameters for D gene detection. It also determines the rearrangement frame and provides a clear summary of search results, including alignment details and match statistics. The tool is robust and can handle various query formats, including FASTA, GenBank, and GI numbers. It is a free public tool with no login requirement. Evaluation of IgBLAST against other tools like iHMMune-align showed that it performs well in identifying germline genes and rearrangements, particularly in clonally related sequences. IgBLAST is a valuable resource for IG sequence analysis, offering a comprehensive and flexible solution for researchers in immunology and bioinformatics.