The identity of the young man in Jacopo Pontormo's Portrait of a Halberdier has long been disputed by art historians. Two probable candidates have been identified: Francesco Guardi, a Florentine nobleman, and Cosimo I de' Medici, Duke of Florence from 1537 and Grand Duke of Tuscany from 1569. The present study is intended to help resolve this controversy by providing evidence, based on pathological criteria, for the identification of Pontormo's Halberdier. Pathological anomalies of the finger joints in the left hand of the Halberdier are compared with those in three acknowledged portraits of Cosimo I de' Medici. These three portraits show the hands of Cosimo in approximately the same position as the Halberdier's hand, thus allowing a meaningful comparison. Detailed observation indicates that there is a close correspondence between the Halberdier's hand deformity and that of Cosimo I in the comparison portraits. This deformity is also consistent with what is known about the medical history of Cosimo I and with the skeletal remains of his left hand. The pathological evidence supports the conclusion that Pontormo's Halberdier is Cosimo I as a young man and that the painting was done around 1537 when Cosimo first became Duke.