Dawid Markelowicz Janowski (1868 – 1927) was a leading Polish-Jewish chess master. Born in Poland, he settled in Paris around 1890 and began his professional chess career in 1894. He won tournaments in Monte Carlo 1901, Hanover 1902 and tied for first at Vienna 1902 and Barmen 1905, so he was in the world's top half dozen.

Janowski was devastating against the older masters such as Wilhelm Steinitz (+5-2), Mikhail Chigorin (+17-4=4) and Joseph Henry Blackburne (+6-2=2). However, he made minus scores against newer players such as Siegbert Tarrasch (+5-9=3), Frank Marshall (+28-34=18), Akiba Rubinstein (+3-5), Geza Maroczy (+5-10=5) and Carl Schlechter (+13-20=13). He was outclassed by world chess champions Emmanuel Lasker (+4-25=7) and José Raúl Capablanca (+1-9=1), but scored respectably against Alexander Alekhine (+2-3=2).

Janowski played very quickly and was a sharp tactician, and was devasting with the bishop pair. Capablanca annotated some Janowski games with great admiration, and said, "when in form [he] is one of the most feared opponents who can exist." However, Capablanca noted that Janowski was weak in the endgame, but Janowski told him, "I detest the endgame." Marshall said, "he could be tremendously stubborn. Janowski could follow the wrong path with more determination than any man I met!"

He played two matches against Emanuel Lasker for the World Chess Championship: in 1909 he lost 8-2, and in 1910 lost 9.5-1.5. The Janowski Indian Defense is named after him.