Published in the present article is a new sculpture by Benedetto da Maiano: a marble tondo, representing the Madonna and Child with the Young Saint John within a garland of angels' heads, which for several generations belonged to a Florentine family and has passed totally unnoticed until now. It is clear, however, that this work – an original by Benedetto both in its invention and in the sbozzatura (roughmodelling) of the entire surface – was given its final form by an anonymous sculptor of the 16th century. It must therefore have remained unfinished in the workshop of Benedetto at his death (1497), together with many other marbles by him already known to scholars. The majority of these were left untouched, such as the 'Saint Sebastian' in the Oratory of the Misericordia in Florence, the various elements of the 'Coronation of King Alfonso II of Aragon' for Naples (today in Florence, Museo Nazionale del Bargello), and a 'Blessing God the Father' (today in the church of San Severo a Legri, near Calenzano). But other marbles, less legible in their provisional state, and also more suited to being reused for worship in other contexts, were completed by other masters during the following century, as was the case for the monumental 'Madonna and Child' at the Misericordia of Florence, concluded by Battista Lorenzi del Cavaliere in 1574-1575. An examination of the well-known posthumous inventory of Benedetto's workshop reveals the presence of the new 'Madonna', along with the other sculptures already mentioned. The relief is listed as “un tondo bozzato di Nostra Donna di braccia 1 ¼”. The corresponding measurement in the decimal system would amount to something significantly different compared to the diameter of our tondo, which is in fact much larger. However, if one considers the plausible possibility of a small mistake in the transcription of the document, which has survived only through a later copy, and supposes that the original entry may have actually read “braccia 1 ¾”, then the dimensions match perfectly. Among the several marble tondos of the Madonna executed by Benedetto, a genre in which he is to be considered the undisputed master of his time, none measure “braccia 1 ¼”. All these tondos are larger in size, including that of “braccia 1 ¾”. The late dating of the newly discovered tondo, which is immediately suggested by its unfinished condition, is fully confirmed by the composition, in which the young Saint John appears for the first time, and by the style, close to the Maniera Moderna. In fact the discovery of this work provides us the missing link between Benedetto's tondos of the Madonna and those of his pupils Giovanfrancesco Rustici and, above all, Michelangelo. The article also explores the distinctive stylistic connection between the rectangular Maianesque 'Madonna and Child with the Young Saint John' in marble owned by the Monte dei Paschi in Siena and the marble altar frontal with the 'Pietà' under the 'Madonna dell'Ulivo', today in the Cathedral of Prato: the author of both must have been Giovanni da Maiano, brother and assistant of Giuliano and Benedetto, who died prematurely in 1478.