Shell sort is a sorting algorithm, devised by Donald Shell in 1959, that is a generalization of insertion sort, which exploits the fact that insertion sort works efficiently on input that is already almost sorted. It improves on insertion sort by allowing the comparison and exchange of elements that are far apart. The last step of Shell sort is a plain insertion sort, but by then, the array of data is guaranteed to be almost sorted.

The algorithm is an example of an algorithm that is simple to code but difficult to analyze theoretically.