Monothalamea is a grouping of foraminiferans, single celled organisms, traditionally consisting of all foraminifera with single-chambered tests, or external shell. Recent work has shown that the grouping is paraphyletic, and as such does not constitute a natural group; nonetheless, the name monothalamea continues to be used by foraminifera workers out of convenience.

"Monothalamea" traditionally contains two groups, neither of which is currently considered to be monophyletic:

Recent molecular evidence has revealed that the deep-sea xenophyophores are in fact agglutinated, single-chambered foraminifera. Molecular evidence has also revealed that the freshwater protist Reticulomyxa is in fact a naked, testless foraminifera, and as such it has been included with monothalameans in scientific discussion.

A 2013 molecular study using small subunit rDNA concluded that known monothalameans made up at least 22 distinct living clades from marine environments with an additional four clades from freshwater eDNA.