Allogromiids, organic-walled foraminifera, are common members of foraminiferal associations in reef and back-reef settings of the Florida Keys and other locales, and many live in cryptic or otherwise protected microhabitats associated with macroalgae and seagrasses. A new species of Allogromia was isolated from the alga Dasycladus vermicularis (Scopoli) collected from prop-roots of the red mangrove, Rhizophora mangle (Linneaus), in Zane Grey Creek, Long Key, Florida. This species, Allogromia arnoldi n. sp., genetically matches sequences of undescribed and uncharacterized allogromiids deposited in GenBank from Cyprus and Jamaica. Allogromia arnoldi is genetically and morphologically distinct from A. laticollaris Arnold, A. laticollaris strain CSH, A. sp. NF (Lee & Pierce, 1963), environmental DNA sequences of morphologically unknown allogromiids, a number of freshwater forms, and several undescribed marine allogromiids known in the literature by nicknames (e.g., “squatter,” “rubble dome,” “twinkle”). This new species of Allogromia belongs to Clade M of the monothalamid foraminifera. Allogromia arnoldi appears to be broadly distributed, and it is fairly easy to isolate and rear in culture. Individuals typically have bright orange cytoplasm, a variable number of apertures, and can assume a wide range of shapes. Transmission electron microscopy (TEM) following high-pressure freezing and freeze substitution shows that the test has a complex fine structure that varies in thickness. It consists of two layers: a thick inner layer with a “herringbone” fine structure, and a thinner, outer electron-opaque mesh-like layer that occurs in patches. The number of nuclei varies per individual, and the non-reproductive nucleus is morphologically similar to that of many other monothalamid foraminifera.