Karl Ernst Adolf Anderssen (6 July 1818 – 13 March 1879) was a German chess master. He won the great international tournaments of 1851 and 1862, but lost matches to Paul Morphy in 1858, and to Wilhelm Steinitz in 1866. Accordingly, he is generally regarded as having been the world's leading chess player from 1851 to 1858, and leading active player from 1862 to 1866, although the title of World Chess Champion did not yet exist.

Anderssen became the most successful tournament player in Europe, winning over half the events he entered, including the very strong Baden-Baden 1870 chess tournament. He achieved most of these successes when he was over the age of 50.

Anderssen is famous today for his brilliant sacrificial attacking play, particularly in the Immortal Game (1851) and the Evergreen Game (1852). He was an important figure in the development of chess problems, driving forward the transition from the "Old School" of problem composition to the elegance and complexity of modern compositions. He was also one of the most likeable of chess masters and became an elder statesman of the game, to whom others turned for advice or arbitration.

Anderssen was born in Breslau (now called Wrocław, Poland), in the Prussian Province of Silesia, in 1818. He lived there for most of his life, sharing a house with and supporting his widowed mother and his unmarried sister. Anderssen never married. He graduated from the public gymnasium (high school) in Breslau and then attended university, where he studied mathematics and philosophy. After graduating in 1847 at the age of 29, he took a position at the Friedrichs-Gymnasium in Breslau as an instructor and later as professor of Mathematics. Anderssen lived a quiet, stable, responsible, respectable middle-class life. His career was teaching mathematics, while his hobby and passion was playing chess.

When Anderssen was nine years old, his father taught him how to play chess. Anderssen said that as a boy, he learned the strategy of the game from a copy of William Lewis' book Fifty Games Between Labourdonnais and McDonnell (1835).

Anderssen first came to the attention of the chess world when he published Aufgabe für Schachspieler ("Task for chess players"), a collection of 60 chess problems, in 1842. He continued to publish problems for many years, both in magazines and as a second collection in 1852. These brought him to the attention of the "Berlin Pleiades" group, which included some of the strongest players of the time, and he played matches against some of them. Anderssen's development as a player was relatively slow, largely because he could spare neither the time nor the money to play many matches against strong players. Nevertheless, by 1846 he was able to put up a good fight against another Pleiades member, Tassilo von Heydebrand und der Lasa, who may have been the world's strongest player at the time. In 1846, he became the editor of the magazine Schachzeitung der Berliner Schachgesellschaft (later called Deutsche Schachzeitung) when its founder Ludwig Bledow, one of the Berlin Pleiades, died. Anderssen held this post until 1865.

In 1848, Anderssen drew a match with the professional player Daniel Harrwitz. On the basis of this match and his general chess reputation, he was invited to represent German chess at the first international chess tournament, to be held in London in 1851. Anderssen was reluctant to accept the invitation, as he was deterred by the travel costs. However, the tournament's principal organizer, Howard Staunton, offered to pay Anderssen's travel expenses out of his own pocket if necessary, should Anderssen fail to win a tournament prize. Anderssen accepted this generous offer.

Anderssen's preparations for the 1851 London International Tournament produced a surge in his playing strength: he played over 100 games in early 1851 against strong opponents including Carl Mayet, Ernst Falkbeer, Max Lange and Jean Dufresne. The 1851 International Tournament was a knock-out event in which pairs of competitors played short matches, and Anderssen won it by beating Lionel Kieseritzky, József Szén, Staunton and Marmaduke Wyvill – by margins of at least two games in every case. His prize was two-thirds of the total prize fund of £500, i.e. about £335; that is equivalent to about £240,000 ($370,200) in 2006's money. When Anderssen and Szén found they were to play each other, they agreed that, if either won the tournament, the other would receive one-third of the prize; this does not appear to have been considered in any way unethical.