Lotus 1-2-3 is a discontinued spreadsheet program from Lotus Software (later part of IBM). It was the first killer application of the IBM PC, was hugely popular in the 1980s, and significantly contributed to the success of IBM PC-compatibles in the business market.

The first spreadsheet, VisiCalc, had helped launch the Apple II as one of the earliest personal computers in business use. With IBM's entry into the market, VisiCalc was slow to respond, and when they did, they launched what was essentially a straight port of their existing system despite the greatly expanded hardware capabilities. Lotus's solution was marketed as a three-in-one integrated solution: it handled spreadsheet calculations, database functionality, and graphical charts, hence the name "1-2-3", though how much database capability the product actually had was debatable, given the sparse memory left over after launching 1-2-3. It quickly overtook VisiCalc, as well as Multiplan and SuperCalc, the two VisiCalc competitors.

Lotus 1-2-3 was the state-of-the-art spreadsheet and the standard throughout the 1980s and into the early 1990s, part of an unofficial set of three stand-alone office automation products that included dBase and WordPerfect, to build a complete business platform. Lotus Software had their own word processor named Lotus Manuscript, which was to some extent acclaimed in academia, but did not catch the interest of the business, nor the consumer market. With the acceptance of Windows 3.0 in 1990, the market for desktop software grew even more. None of the major spreadsheet developers had seriously considered the graphical user interface (GUI) to supplement their DOS offerings, and so they responded slowly to Microsoft's own GUI-based products Excel and Word. Lotus was surpassed by Microsoft in the early 1990s, and never recovered. IBM purchased Lotus in 1995, and continued to sell Lotus offerings, only officially ending sales in 2013.