BS2000 is an operating system for IBM 390-compatible mainframe computers developed in the 1970s by Siemens (Data Processing Department EDV) and from early 2000s onward by Fujitsu Technology Solutions.

This uniformity of the user interface and the entire BS2000 software configuration makes administration and automation particularly easy.

Currently, it is mainly used in Germany - making up to 83% of its total user base - (63% of all SE/S Servers are installed at the German Travel Association (DRV) and German Tax Authority) as well as in the United Kingdom (8%), Belgium (4.8%) and other European countries (Austria, Czech Republic, Denmark, Italy, Luxembourg, Netherland, Spain, Switzerland) (4.2%).

From the outset TSOS also allowed data peripherals to be accessed only via record- or block-oriented file interfaces, thereby preventing the necessity to implement device dependencies in user programs.

The same operating system was also sold to Sperry Univac when it bought most of RCA's computer division.

With the advent of the VM2000 virtual machine in 1990, multiple BS2000 systems, of the same or different versions, can run in parallel on the same computer.

In 2002, BS2000/OSD was ported to the SPARC architecture, leading to the Fujitsu Siemens Computers' SX server line.