Pfaffnau is a municipality in the district of Willisau in the canton of Lucerne in Switzerland. In the municipality two different parts are existing, the village Pfaffnau and the Former Monastery of St. Urban.
Pfaffnau is first mentioned in 924 as Fafanhaa/Fafana. In 1173 it was mentioned as Phafena.
Pfaffnau has an area of 17.62 km2 (6.80 sq mi). Of this area, 11.41 km2 (4.41 sq mi) or 64.8% is used for agricultural purposes, while 4.3 km2 (1.7 sq mi) or 24.4% is forested. Of the rest of the land, 1.71 km2 (0.66 sq mi) or 9.7% is settled (buildings or roads), 0.09 km2 (22 acres) or 0.5% is either rivers or lakes and 0.04 km2 (9.9 acres) or 0.2% is unproductive land.
Of the built up area, housing and buildings made up 5.3% and transportation infrastructure made up 2.6%. Out of the forested land, 23.0% of the total land area is heavily forested and 1.4% is covered with orchards or small clusters of trees. Of the agricultural land, 41.5% is used for growing crops and 21.6% is pastures, while 1.7% is used for orchards or vine crops. All the water in the municipality is flowing water.