The Piaggio Sì is a moped that was produced by the Italian manufacturer Piaggio.

It received homologation on 20 November 1978 and was presented in Genoa on 1 February 1979.

In the North American market it was sold by the American division of Piaggio, the Vespa of America Corporation, as Piaggio Si.

With engine derived from the contemporary Piaggio Ciao, it differed from it for the fork telescopic front, rear monoshock instead of rigid frame, for the headlights front and rear of different design, for the longer saddle, more comfortable and equipped with a small compartment under the saddle, and for the 4-spoke wheels.

As for the engine, it differed only in the cylinder head of different design and with longer cooling fins. Furthermore, there is a version equipped with Variomatic, a solution that allows you to overcome slopes more easily by increasing the reduction ratio of the transmission system.

The Sì, on sale in Italy since February 1979, was available with both long saddle and short saddle and spoke or alloy wheels. This model, until about mid-1979 was only available in two colors: Light Gray Metallic 108 and Beige 560. Subsequently, around mid-1979, the Blue Marine color was added. The muffler was square in section. It was available with variator (SIV) and pulley (SIM). The engine had the traditional ignition with points. The brake levers were made of metal and ended in a ball. In April/May 1980 the mud flap was added and from 1981/82 the brake levers became plastic. In 1981 the horn button also changed, which until then had been a gray casing and then became all black. Since 1983, the short saddle and spoked wheels were optional. From 1984 the optional electronic engine was introduced. From 1984/85 the engine cradle was no longer in body color but black/gray.

It differs from the first series for the upside-down and advanced hydraulic fork as in the Piaggio Bravo, and the double rear shock absorber.

It differs from the Montecarlo in various red painted details.