A Y-STR is a short tandem repeat (STR) on the Y-chromosome. Y-STRs are often used in forensics, paternity, and genealogical DNA testing. Y-STRs are taken specifically from the male Y chromosome. These Y-STRs provide a weaker analysis than autosomal STRs because the Y chromosome is only found in males, which are only passed down by the father, making the Y chromosome in any paternal line practically identical. This causes a significantly smaller amount of distinction between Y-STR samples. Autosomal STRs provide a much stronger analytical power because of the random matching that occurs between pairs of chromosomes during the zygote-making process.

Y-STRs are assigned names by the HUGO gene nomenclature committee (HGNC).

Some testing companies have different formats for the way STR markers are written. For example, the marker DYS455 may be written as DYS455, DYS 455, DYS#455, or DYS# 455. The scientific standard accepted by HUGO and NIST is DYS455.

DYS is a variation on the jargon used in human autosomal STR testing where the second character is typically reserved for the chromosome number (e.g. D8S1179).

D = DNA
Y = Y-chromosome
S = (unique) segment

There are regions on DNA that are made up of multiple copies of short repeating sequences of bases (for example TATT) which repeat a variable number of times depending on the individual. These regions, called "variable number short tandem repeats", are what is looked at when performing STR analysis. The likelihood of two people having the same number of repeated sequences is extremely small, and becomes even smaller the more regions that are analyzed. This makes up the basis of short tandem repeat analysis. The cornerstone for this process, however, is polymerase chain reaction (PCR). This allows forensic scientists to make millions of copies of the STR regions. Gel electrophoresis then "yields the number of times each repeat unit appears in the fragment." This allows for easy comparison of DNA.

Y-STR analysis is not a robust method of identity determination due to the possibility of haplotype convergence, whereby two or more men acquire the same Y-STR repeat numbers purely by chance rather than by common descent. Some lineages in the R1b Y haplogroup (the most common in Europe) are a prominent example of this.

In the United States, 13 different autosomal STR loci are used as a basis of analysis for forensic purposes. If crime scene DNA is ample and all 13 autosomal loci accessible, the likelihood of two unrelated people matching the same sample is around one in one billion.