Which mammal is the smallest known extant mammal by mass?

The Etruscan shrew is the smallest known extant mammal by mass.

Suncus etruscus usually weighs about 1.8 grams, with recorded body masses roughly between 1.3 and 2.5 grams. It is found across warm regions from parts of Europe and North Africa through Asia, often in shrubby habitats, gardens, orchards and woodland edges.

Such a tiny body comes with an extreme metabolism. The shrew may eat 1.5 to 2 times its own body weight each day, mainly consuming insects and other small invertebrates. Its heart rate can reach about 1,500 beats per minute, helping support its rapid activity.

The usual confusion is with the bumblebee bat. That bat is often regarded as the smallest mammal by skull size and body length, but the Etruscan shrew holds the mass record among living mammals.