Niccolò di ser Sozzo (Siena, ... – Siena, 1363) was an Italian painter and illuminator. The superb illustration of the Assumption of the Virgin in the Caleffo Bianco (a register of state documents in the Siena Archive) and the polyptych of the Virgin and Child with Four Saints (Siena, Pinacoteca Nazionale) are signed by Niccol? di ser Sozzo. The polyptych is also signed by Luca di Tomm? with the date 1362. In 1348 Niccol? was in debt to the Siena Comune, and in 1363 he was mentioned in the city's Libro delle Arti. He has been associated with documents referring to services undertaken on behalf of the Siena Comune, but these relate to the son of a notary, ser Sozzo di Francesco Tegliacci. The painter of the signed miniature and polyptych was probably the son of an obscure Sienese illuminator, ser Sozzo di Stefano.
The Caleffo is a copy of an official act of a municipality, kept separately from the originals to be able to have on hand. This name, of uncertain etymology, was used in Siena in the Middle Ages. Because of this full-page and brightly coloured image of the Virgin in Assumption, the present manuscript is almost exclusively referred to as the Caleffo dell'Assunta. The frontispiece of the manuscript was, for a time, the defining work attributed to its illuminator, Niccolo di Ser Sozzo. This illumination has much in common with Sienes masterworks like Simone Martini's Maesta or Ambrogio Lorenzetti's Allegory of Good and Bad Government.
The picture shows an illuminated page of the Caleffo dell'Assunta. The illumination, made by Niccolò di Ser Sozzo, a skilled painter having expertise specialised in the production of manuscripts, represents the Virgin of the Assumption with St Thomas receiving the girdle, and Sts Crescenzio, Victor, Savino and Ansano. 
Pagg. 555-556 Michael Bryan, Dictionary of Painters and Engravers, Biographical and Critical (Volume II L-Z), York St. #4, Covent Garden, London; Original from Fogg Library, Digitizzato 18 maggio 2007, Walter Armstrong & Robert Edmund Graves, 1889.