WK1 is a spreadsheet document file format used by no longer developed software Lotus 1-2-3,which was included in Lotus Smart Suite software productivity suite. WK1 documents were very popular in 1990s when Lotus products were competing with Microsoft office tools.

WK1 files store data using the standard spreadsheet structure with cells organized in rows in columns. WK1 format also supports graphs, charts, formulas, and functions. It is possible to open Lotus document files with free office software OpenOffice.