Henricus Martellus Germanus is the latinized name of Heinrich Hammer (Italian: Enrico Martello), a geographer and cartographer from Nuremberg who lived and worked in Florence from 1480 to 1496.
Between around 1489 and 1491, he produced at least one world map which is remarkably similar to the terrestrial globe produced by Martin Behaim around 1492, the Erdapfel. Both show novel adaptions of the existing Ptolemaic model, opening a passage south of Africa and creating an enormous new peninsula east of the Golden Chersonese (Malaysia). Both possibly derive from maps created around 1485 in Lisbon by Bartolomeo Columbus.
The only extant manuscript world map, measuring 201 by 122 centimetres (79 in × 48 in) in size, was rediscovered in the 1960s and donated to Yale's Beinecke Rare Book and Manuscript Library. An inscription in the lower left corner states: "Although Strabo and Ptolemy and the majority of the ancients were most assiduous in describing the world we, however, bring together in this picture and carefully show in their true places the new knowledge that escaped their diligence and remained unknown to them".
Martellus’ map served as an inspiration for the Waldseemüller map of 1507. The overall layout was similar, and Martin Waldseemüller used the same projection as Martellus, the pseudo-cordiform projection. Both cartographers added decorative wind-heads in the borders of their maps, and both also took advantage of the extra space in the lower corners of the maps created by the swooping lines of the projection to add text blocks in those corners. The shape of northern Africa is the same on both maps, that is, it is Ptolemaic with a sharp northwestern corner. The shape of eastern Asia is similar on the two maps, with a huge peninsula jutting southwestward into the Indian Ocean, and Japan is in precisely the same position on the two maps, at the eastern edge.
Martellus also produced an Insularium Illustratum ("Illustrated Book of Islands") of which four manuscripts are extant, plus one draft in Biblioteca Laurentiana. It contains an illustrated description of islands of the Aegean Sea, mostly copied from a previous work by Cristoforo Buondelmonti, plus maps from other islands, several regional maps and a world map.
He has been identified with an Arrigho di Federigho who authored the first translation into German of Bocaccio's Decamerone. According to this theory, the surname Martellus would come from the Martelli family, to which Henricus / Arrigho was linked.