WATFIV (Waterloo FORTRAN IV), developed at the University of Waterloo, Canada is an implementation of the Fortran computer programming language. It is the successor of WATFOR.

WATFIV was used from the late 1960s into the mid-1980s. WATFIV was in turn succeeded by later versions of WATFOR. Because it could complete the three usual steps ("compile-link-go") in just one pass, the system became popular for teaching students computer programming.

In the early 1960s, newly formed computer science departments started university programs to teach computer programming languages. The Fortran language had been developed at IBM, but suffered from slow and error-prone three-stage batch processing workflow. In the first stage, the compiler started with source code and produced object code. In the second stage, a linker constructed a complete program using growing libraries of common functions. Finally, the program was repeatedly executed with data for the typical scientific and business problems of customers. Each step often included a new set of punched cards or tape. Students, on the other hand, had very different requirements. Their programs were generally short, but usually contained logic and syntax errors, resulting in time-consuming repetition of the steps and confusing "core dumps" (It often took a full day to submit and receive the successful or failed output from the computer operator). Once their programs worked correctly, they were turned in and not run again.

In 1961, the University of Wisconsin developed a technology called FORGO for the IBM 1620 which combined some of the steps. Similar experiments were carried out at Purdue University on the IBM 7090 in a system called PUFFT.

In summer 1965, four undergraduate students of the University of Waterloo, Gus German, James G. Mitchell Richard Shirley and Robert Zarnke, led by Peter Shantz, developed a Fortran compiler for the IBM 7040 computer called WATFOR. Its objectives were fast compilation speed and effective error diagnostics at both compile and execution time. It eliminates the need for a separate linking step and, as a result, FORTRAN programs which contain no syntax errors are placed into immediate execution. Professor J. Wesley Graham provided leadership throughout the project.

This simple, one-step process allowed non-experienced programmers to learn programming with lower cost in time and computing resources. To aid in debugging, the compiler uses an innovative approach to checking for undefined variables (an extremely common mistake by novice programmers). It uses a diagnostic feature of the 7040 that can deliberately set areas of memory to bad parity. When a program tries to reference variables that hadn't been set, the machine takes an interrupt (handled by the Watfor runtime routines) and the error is reported to the user as an undefined variable. This has the pleasant side effect of checking for undefined variables with essentially no CPU overhead.

WATFOR quickly gained popularity and over 75 institutions installed it on their IBM 7040 systems. The distribution of the compiler was handled by Sandra Bruce (née Hope).

In 1966, the University planned to replace the 7040 with an IBM System/360 computer, which was much faster but not software compatible. A team of full-time employees and undergraduate students was formed to write an IBM 360 version. The project members, Betty Schmidt, Paul Dirksen, Paul H. Cress, Lothar K. "Ned" Kesselhut, Bill Kindree and Dereck Meek, were later joined by Mike Doyle, Rod Milne, Ron Hurdal and Lynn Williams, completed 360 WATFOR in the early part of 1967. Many other institutions (universities, colleges, businesses and governmental agencies) started using the WATFOR compiler to meet needs similar to those experienced at the University of Waterloo. The distribution of the software and customer support was carried on by Sandra Ward.