Monteggio is a former municipality in the district of Lugano in the canton of Ticino in Switzerland. On 18 April 2021 the municipalities of Croglio, Monteggio, Ponte Tresa and Sessa merged to form Tresa.

Monteggio is first mentioned in 1466 as da Montegio. Until 1819 the current village core was called Albio. The name comes from the fortified seat of the Capitanei of Sessa, the ruins of which are on the site of the Chapel of S. Adalberto (first mentioned in 1428) located in Castello. During the Middle Ages the history of Monteggio was closely connected to the Sessa family. The communities in the Valley of Tresa (Monteggio, Ponte Tresa and Croglio) were given special privileges, because they had special military and defensive obligations. The tax exemptions granted in the 14th century by the Duke of Milan were reconfirmed in 1513 by the Swiss Confederation.

Religiously, Monteggio belongs to the parish of Sessa.

In the past, many industries operated in Monteggio, including; brickworks, mills, hammer mills, fishing and a sawmill. By the middle of the 19th century, peat was cut near the village and there was a foundry. In the past few decades, various commercial and industrial enterprises have settled in the municipality.

Monteggio had an area, as of 1997^[update], of 3.36 square kilometers (1.30 sq mi). Of this area, 1.36 km² (0.53 sq mi) or 40.5% is used for agricultural purposes, while 1.64 km² (0.63 sq mi) or 48.8% is forested. Of the rest of the land, 0.65 km² (0.25 sq mi) or 19.3% is settled (buildings or roads), 0.09 km² (22 acres) or 2.7% is either rivers or lakes and 0.07 km² (17 acres) or 2.1% is unproductive land.

Of the built up area, housing and buildings made up 13.4% and transportation infrastructure made up 2.4%. Power and water infrastructure as well as other special developed areas made up 2.1% of the area Out of the forested land, 43.5% of the total land area is heavily forested and 5.4% is covered with orchards or small clusters of trees. Of the agricultural land, 22.9% is used for growing crops, while 4.2% is used for orchards or vine crops and 13.4% is used for alpine pastures. All the water in the municipality is flowing water. Of the unproductive areas, all of it is unproductive vegetation.

The former municipality is located in the Lugano district, in the lower Malcantone valley on the Italian border. It is broken up into 26 sections or settlements including Brusata, Castello, Fornasette, Lisora, Molinazzo and Ponte Cremenaga.

The blazon of the municipal coat of arms is Per saltire gules and sable and in a chief or an eagle displayed sable.