The white clam, Amarilladesma mactroides, is a sandy bivalve naturally found in the infralittoral zone of beaches from Rio de Janeiro, Brazil, to northern Argentina. This species has been subjected to intense environmental and anthropogenic pressures in recent years, causing several populations to collapse. Although studies recommend captive juvenile production, laboratory breeding technology is not yet available, which prevents the assessment of the feasibility of restocking natural environments or aquaculture. Therefore, this dissertation aimed to estimate the reproductive period of female A. mactroides in the extreme south of Brazil through ovarian development analysis using histological tools combined with biochemical composition variation determination. From October 2021 to September 2022, monthly collections of at least 100 clams were performed at Cassino Beach, Rio Grande, RS, southern Brazil. Measurements included length, height, width, total wet weight (shells and soft tissue), wet weight of shells and soft tissue, dry weight of soft tissue and shells; crude protein, total lipids, glycogen, and lactate analyses were also performed. Additionally, histological analyses of gonads, including oocyte diameter, and immunological assays were conducted. Results indicate that in the extreme south of Brazil, the white clam exhibits two reproductive periods: a more intense one in spring (October and November) and a less intense one in autumn-winter (from May-June to September). The determination of the reproductive period was made possible mainly through histological analysis of the ovaries, especially by measuring the oocyte diameter. The condition index and concentrations of total lipids and glycogen in the soft tissues also proved to be useful tools for characterizing the reproduction of this important bivalve species.