The river Ticino (/tɪˈtʃiːnoʊ/ tih-CHEE-noh, Italian: [tiˈtʃiːno]; Lombard: Tesin; French and German: Tessin; Latin: Ticīnus) is the most important perennial left-bank tributary of the Po. It has given its name to the Swiss canton through which its upper portion flows.

It is one of the four major rivers taking their source in the Gotthard region, along with the Rhône, Reuss and Rhine. The river rises in the Val Bedretto in Switzerland at the frontier between the cantons of Valais and Ticino right below the Nufenen Pass, is fed by the glaciers of the Alps and later flows through Lake Maggiore, which traverses the border to Italy. The Ticino joins the Po a few kilometres downstream (along the Ticino) from Pavia. It is about 248 kilometres (154 mi) long. The stretch of river between Lake Maggiore and the confluence in the Po is included in the Parco naturale lombardo della Valle del Ticino, a Nature reserve included by UNESCO in the World Network of Biosphere Reserves.

The name may have meant "the runner," from Proto-Indo-European *tekʷ-ino-s, from *tekʷ- (“to run, flow”).

The Ticino arises from the many torrents that drain the mountain flanks on the eastern side of Nufenen Pass in the upper Val Bedretto. From here, the river flows to the north-east through the Val Bedretto to Airolo (1,127 metres (3,698 ft)) where the south ramp of the St Gotthard Pass and south portals of both the Gotthard Railway Tunnel and the Gotthard Road Tunnel are located. Then it turns slowly to the southeast while entering the Valle Leventina. In the Leventina, the Ticino flows through three terrain steps separated by two narrow gorges (Gola di Monte Piottino and Biaschina) and finally reaches the canton's floor near Bodio at around 320 metres (1,050 feet). Shortly after, at Biasca (286 metres (938 feet)), the river is joined by its first major tributary, the Brenno which flows from Passo del Lucomagno through the Valle di Blenio.

Before the Ticino passes west of the canton's capital, Bellinzona, the river converges with its second larger left tributary, the Moesa, originating below the Passo del San Bernardino and flowing through the Valle Mesolcina. The Valle Mesolcina (and the Val Calanca) belongs geographically and culturally to the Ticino, but politically it is part of the canton of the Grisons. This part of the river valley between Biasca and Bellinzona is called the Riviera. The river is dammed after Bellinzona. Now the Ticino turns almost westwards and flows through the perfectly flat Piano di Magadino, an important cultivation and nature reserve area the river generated itself by its own sediments over ten thousands of years, before it enters the Lago Maggiore at the lowest point of Switzerland at 193 metres (633 ft).

Only about the first sixth of the lake lies in Switzerland. Four major tributaries join the Ticino in Lago Maggiore. The Verzasca and the Maggia enter the lake in Switzerland in Tenero and in Locarno/Ascona, respectively. The Tresa drains the Lago di Lugano and flows in Luino into Lago Maggiore. The Toce drains many valleys to the northwest of Lago Maggiore and joins the lake near Feriolo. It surpasses the Ticino in average discharge volume and contains the highest peak of the catchment area of the Ticino river system, the Grenzgipfel (4,617 metres (15,148 feet)) in the Monte Rosa massif.

In Sesto Calende the river, now called Fiume Ticino, exits Lake Maggiore and meanders southwards, passing the Milan Malpensa Airport in the east. Now slowly turning to the southeast, the river circumvents Milan at a distance of about 20 kilometres (12 mi) in the east. Shortly before it converges with the Po in the south of Milan, it passes Pavia on its northern shore.

The legendary Gallic leader Bellovesus was said to have defeated the Etruscans here in circa 600 BC. Ticino was the location of the Battle of Ticinus, the first battle of the Second Punic War fought between the Carthaginian forces of Hannibal and the Romans under Publius Cornelius Scipio in November 218 BC.