The ProCo RAT is a distortion pedal produced by ProCo Sound for use with electric guitar. The RAT has become one of the industry's most popular effects pedals due to its tone and versatility, producing an aggressive style of distortion that can cross over into overdrive or fuzz depending on the pedal's settings. Prototypes were designed by ProCo engineer Scott Burnham in the company's rat-infested basement in Kalamazoo, Michigan, with early units made to order for customers and full production starting in 1979. The current production model, the RAT2, was released in 1988 and multiple variations of the circuit have since been released by ProCo, such as the TurboRAT and You Dirty RAT. Many other pedal companies produce clones of or homages to the RAT circuit. Guitar World credited the RAT with creating "the template for what distortion sounds like to this day."

ProCo engineer Scott Burnham began designing what became the RAT circuit in the mid-1970s, possibly as early as 1974, with the assistance of fellow engineer Steve Kiraly. This was around the same time "hard-clipping" distortion pedals like the MXR Distortion+ were hitting the market. While experimenting with his design, Burnham accidentally used a 47 ohm resistor instead of a 470 ohm resister value to bias the op-amp, resulting in much higher levels of gain. Combined with a diode clipping section, this allowed for more aggressive distortion than was available at the time. Author Pagan Kennedy describes Burnham's accidental invention:

"One day, as he was soldering parts, he picked up a resistor of the wrong size—this was his lucky mistake—and attached it to a circuit board. The machine began shrieking and moaning. And that's how he heard a new sound; it was beautiful, haunting, ugly, and full of soul. He immediately recognized this as a major discovery, and he built that sound into a distortion pedal he named the Rat. Attached to a guitar, the Rat turned every note into a blast of outrage."

The earliest physical prototypes of the RAT—named for the rodent-infested basement of the ProCo facility in Kalamazoo, Michigan it was designed in—were made in 1977 or 1978. These dozen or so units used premade Bud Box enclosures, which led to them being known as "Bud Box RATs," and they were made-to-order for customers. ProCo owner Charlie Wicks liked the RAT enough to make it a full production model and it first appeared in 1979's ProCo product catalog.

ProCo's first production model RAT used a large, wrap-around metal enclosure painted black with white, silk-screened text and logos. The earliest of these so-called "Big Box" RATs went through multiple cosmetic changes in 1979: the logos changed slightly and different knobs were used. In 1981, ProCo made its first design change to the RAT, albeit a small one. ProCo reversed the contact points on the PCB connecting to the tone control's potentiometer to reverse the direction of its sweep so that the pedal's tone became darker as the control was turned clockwise. The "Tone" control label was also changed to "Filter."

Living Colour's Vernon Reid, a "Big Box" RAT user, said of the music scene's reaction to the RAT, "People were freaking out. Hard-rock dudes, metal dudes, funk dudes—everybody. It was insane." Reid described the RAT as producing "amplifier-like overdrive but even more extreme" and noted that it could make a clean Fender amp sound like a Marshall or Mesa/Boogie.

By the early 1980s, ProCo was seeing RAT sales drop as competitors like Boss, MXR, and DOD offered pedals in smaller, pedalboard-friendly enclosures. ProCo sought to keep up by releasing a compact version of the RAT in 1984. Cosmetically, the new RAT used a block logo that was white with black text inside it and it became known as the "White Face" RAT. The new logo was difficult to produce however and ProCo opted to change the design in 1986 so that the white block was reduced to a thin border around white "RAT" text in an otherwise black logo. Consequently, this update became known as the "Black Face" RAT.

In 1988, the RAT2 was introduced to replace the original RAT model. The RAT2 included an LED on/off status light and the screen-printed labels were replaced with glow-in-the-dark mylar. The RAT2 remained popular enough among players that ProCo was struggling to keep up with demand by the early 2000s. To ease his factory's workload, Wicks struck a deal with parts supplier Neutrik to produce the RAT2 in Neutrik's Chinese factory. These overseas-made RAT2s came to market in 2008 with sloped enclosures (introduced with the TurboRAT) and using the OD07DP op-amp, which ProCo had replaced the original LM308 with by 1996 since the LM308 had been declared obsolete by Texas Instruments.