Siegbert Tarrasch (German pronunciation: [ˈziːɡbɐt ˈtaraʃ]; 5 March 1862 – 17 February 1934) was a German chess player, considered to have been among the strongest players and most influential theoreticians of the late 19th and early 20th century.

Tarrasch was born in Breslau, in what was then Prussian Silesia and now is Poland. Having finished school in 1880, he left Breslau to study medicine in Berlin and then in Halle. With his family, he settled in Nuremberg, Bavaria, and later in Munich, setting up a successful medical practice. He had five children. Tarrasch was Jewish, converted to Christianity in 1909, and was a patriotic German who lost a son in World War I, yet he faced antisemitism in the early stages of the Third Reich.

A medical doctor by profession, Tarrasch may have been the best player in the world in the early 1890s. He scored heavily against the ageing World Champion Wilhelm Steinitz in tournaments (+3−0=1) but refused an opportunity to challenge Steinitz for the world title in 1892 because of the demands of his medical practice.

Soon afterwards, in St. Petersburg in 1893, Tarrasch drew a hard-fought match against Steinitz's challenger Mikhail Chigorin (+9−9=4) after leading most of the way. He also won four major tournaments in succession: Breslau 1889, Manchester 1890, Dresden 1892, and Leipzig 1894.

However, after Emanuel Lasker became world chess champion in 1894, Tarrasch could not match him. Fred Reinfeld wrote: "Tarrasch was destined to play second fiddle for the rest of his life." For example, Lasker scored much better against mutual opponents, e.g. vs. Chigorin, Tarrasch had +2 over 34 games while Lasker scored +7 in 21; vs. Akiba Rubinstein, Tarrasch was −8 without a single win, while Lasker scored +2−1=2; vs. David Janowski, Tarrasch scored +3 compared to Lasker's huge +22; vs. Géza Maróczy, Tarrasch was +1 over 16 games while Lasker scored +4−0=1, vs. Richard Teichmann Tarrasch scored +8−5=2, while Lasker beat him all four tournament games. However, Tarrasch had a narrow plus score against Harry Nelson Pillsbury of +6−5=2, while Lasker was even +5−5=4. Still, Tarrasch remained a powerful player, demolishing Frank Marshall in a match in 1905 (+8−1=8), and winning Ostend 1907 over Schlechter, Janowski, Marshall, Burn, and Chigorin.

By the time Lasker finally agreed to give Tarrasch a world title match in 1908, there was no love lost between them. The story goes that when they were introduced at the opening of their 1908 championship match, Tarrasch clicked his heels, bowed stiffly, and said, "To you, Dr. Lasker, I have only three words, check and mate"—then left the room. Lasker would beat Tarrasch convincingly in their match +8−3=5.

Tarrasch continued to be one of the leading players in the world. He finished fourth in the very strong St. Petersburg 1914 chess tournament, behind only World Champion Lasker and future World Champions José Raúl Capablanca and Alexander Alekhine, and ahead of Marshall, Ossip Bernstein, Rubinstein, Nimzowitsch, Blackburne, Janowski, and Gunsberg. His win against Capablanca in the 19th round, though much less famous than Lasker's win against Capablanca the round before, was essential to enable Lasker to achieve his famous come-from-behind victory over Capablanca in the tournament. This tournament was probably Tarrasch's swan song, because his chess career was not very successful after this, although he still played some highly regarded games.

Tarrasch lost +0-5=1 to Lasker in a 1916 match.