The morphological ontogeny of Amerobelba decedens Berlese, 1908 is described and illustrated. In the adult of this species, the bothridial seta is setiform, unilaterally ciliate, notogastral setae are short and barbed, and sexual dimorphism is present on tarsus IV. Most prodorsal setae of juveniles are short and smooth, except for medium-sized ro, barbed in the nymphs. The bothridial seta is long, setiform and barbed in distal part. The gastronotal setae are of medium size and barbed, except for smooth c₁ and c₂ in all juveniles, and smooth of d-series and short h₂ in the larva. Setae c₁ and c₂ are inserted on large macrosclerite, and other gastronotal setae are inserted on individual microsclerites, except for h₂ in the larva. The anal and anogenital regions are reticulate, and adanal setae are inserted on individual microsclerites. The nymphs are quadrideficient and eupheredermous, i.e. they lose setae of d-series and carry the exuvial scalps of previous instars. In this species, seta d accompanies solenidion σ on genu III and φ on tibia III and IV, both in the juveniles and adults. It is a xero-thermophilous species, with one generation per year.