Petrus Apianus (April 16, 1495 – April 21, 1552), also known as Peter Apian, was a German humanist, known for his works in mathematics, astronomy and cartography. The lunar crater Apianus is named in his honour.

He was born as Peter Bienewitz (or Bennewitz) in Leisnig in Saxony; his father was a shoemaker. The family was relatively well off, belonging to the middle class citizenry of Leisnig. Apianus was educated at the Latin school in Rochlitz. From 1516 to 1519 he studied at the University of Leipzig; during this time, he Latinized his name to Apianus (lat. apis means "bee"; "Biene" is the German word for bee).

In 1519, Apianus moved to Vienna and continued his studies at the University of Vienna, which was considered one of the leading universities in geography and mathematics at the time and where Georg Tannstetter taught. When the plague broke out in Vienna in 1521, he completed his studies with a B.A. and moved to Regensburg and then to Landshut.

In Landshut, he produced his Cosmographicus liber (1524), a highly respected work on astronomy and navigation that was to see at least 30 reprints in 14 languages and that remained popular until the end of the 16th century. He married the daughter of a councilman of Landshut, Katharina Mosner, in 1526. They would have 14 children together, five girls and nine sons, one of which was Philipp Apian.

In 1527, Peter Apian was called to the University of Ingolstadt as a mathematician and printer. His print shop started small. Among the first books he printed were the writings of Johann Eck, Martin Luther's antagonist. His print shop soon became well-known for its high-quality editions of geographic and cartographic works.

Through his work, Apian became a favourite of emperor Charles V. Charles had praised his work (the Cosmographicus liber) on the Reichstag of 1530 and granted him a printing monopoly in 1532 and 1534. In 1535, the emperor made Apian an armiger, i.e., granted him the right to display a coat of arms. In 1540, Apian printed the Astronomicum Caesareum, dedicated to Charles V. Charles promised him a truly royal sum (3,000 golden guilders), appointed him his court mathematician, and made him a Reichsritter and in 1544 even a Hofpfalzgraf. All this furthered Apian's reputation as an eminent scientist.

Despite many calls from other universities, including Leipzig, Padua, Tübingen, and Vienna, Apian remained in Ingolstadt until his death. Although he neglected his teaching duties, the university evidently was proud to host such an esteemed scientist. Apian's work included mathematics — in 1527 he published a variation of Pascal's triangle, and in 1534 a table of sines — as well as astronomy. In 1531, he observed a comet and discovered that a comet's tail always point away from the sun. (Girolamo Fracastoro also detected this in 1531, but Apian's publication was the first to also include graphics.) He designed sundials, published manuals for astronomical instruments and crafted volvelles ("Apian wheels"), measuring instruments useful for calculating time and distance for astronomical and astrological applications.