One of the hypothesis suggest that the city received its name on account of the large amount of the edible vegetable found in the area (see mustard).

Another hypothesis listed by the historian Marisa Oliveira Guedes is that the name Mustardas was given not by the amount of the plant, which does not actually exist on the fields, but, because Mostardas were the name of the trenches used during the wars in Portugal, which were covered with a mat made of reed and Taquara bamboo, with camouflage made of mustard, because this plant does not wither.

The book Histórias Antigas de Mostardas, by Jayme Collares Neto, has genealogical information and histories of past inhabitants.

It is available in Mostardas (Mega Informática) and in bookstores in Porto Alegre (Érico Veríssimo, Martins Livreiro), Rio Grande e Pelotas (Vanguarda).

The city is part of the south shore of the state of Rio Grande do Sul, on an isthmus formed by Laguna dos Patos and Atlantic Ocean.