The Fiat 600 (Italian: Seicento, pronounced [ˌsɛiˈtʃɛnto]) is a small, rear-engined city car and economy family car made by Italian carmaker Fiat from 1955 to 1969 — offered in two-door fastback sedan and four-door Multipla mini MPV body styles. The 600 is considered a pop icon of the Italian economic miracle, and the three-row seating Multipla, though diminutive and odd-looking, is seen as one of the first mass-produced minivans.

Measuring just 3.22 m (10 ft 7 in) long, its all-new design was Fiat's first rear engined car, and was priced at 590,000 lire (the equivalent of about €8,680 or US$9,440 in 2023). The total number produced from 1955 to 1969 at the Mirafiori plant in Turin was 2,695,197.

The 1955 Fiat 600 also formed the blueprint for an even smaller sibling, the 2.97 m (9 ft 9 in) 2nd generation "Nuova" (New) Fiat 500, launched two years later – which was, although rounder in shape, largely copied from the 600's layout and design. Later, the 600's platform also formed the basis for the larger 850 saloon, coupé and spider, launched from 1964, which coexisted with the 600 in Fiats line-up for five years, until the 600 was cancelled.

During the 1950s, 1960s, 1970s, and 1980s, many units were built under licence in countries such as Spain (as SEAT 600), where it became the cultural icon of the Spanish miracle and where it was nicknamed Seiscientos; Argentina, where it was nicknamed Fitito (a diminutive: "little Fiat") and former Yugoslavia where it was nicknamed Fića or Fićo (pronounced 'fee-cha' or 'fee-cho' respectively).

Fiat replaced their 500 and 600 with the 126 and 127 models, featuring much more modern, but again very similar styling, however where the 126 carried over much of the 500's underpinnings, the 127 was an all new, slightly more expensive design with a front-engine, front-wheel-drive layout, and possibly a rear hatch. But in Spain, SEAT chose to develop a more affordable successor, the 1974 SEAT 133, updated with a similarly modernised body and interior, but largely reusing the Fiat 600 platform. For a while, Fiat also sold it as the Fiat 133, a cheaper alternative to the 127.

Codenamed Progetto 100 ("Project 100"), the Fiat 600 mirrored the layout of the Volkswagen Beetle and Renault 4CV of its era. Aimed at being an economical but capable vehicle, its design parameters stipulated a weight of around 450 kg with the ability to carry 4 people and luggage with a cruising speed of no less than 85 km/h. A total of 5 prototypes were built between 1952 and 1954, which all differed from each other. Chassis number 000001 with engine number 000002 is believed to be the sole remaining example, according to a recent report by Quattroruote's "Ruoteclassiche" vintage division. It was powered by an innovative single-cam V2-cylinder engine designed to simplify maintenance and did not feature a clutch pedal. At the official launch in 1955, FIAT engineer, Dante Giacosa declared that the aim had been to create something new, both in the interest of progress and simplification. This prototype, however, did not become the chosen design.

The car had hydraulic drum brakes on all four wheels. Suspension was a single double-mounted leafspring—which acts as a stabilizer—between the front wheels coupled to gas-charged shock absorbers, and an independent coil-over-shock absorber setup coupled to semi-trailing arms at the rear. All 600 models had 3-synchro (no synchro on 1st) 4-speed transaxles. Unlike the Volkswagen Beetle or Fiat 500, the Fiat 600 is water-cooled with an ample cabin heater and, while cooling is generally adequate, for high-power modified versions a front-mounted radiator or oil cooler is needed to complement the rear-mounted radiator. All models of the 600 had generators with mechanical external regulators.

The top speed ranged from 95 km/h (59 mph) empty with the 633-cc inline-four engine to 110 km/h (68 mph) with the 767-cc version. The car had good ventilation and defrosting systems.^{[citation needed]}