Forty-three (43, XLIII) is the natural number following forty-two and preceding forty-four.

Forty-three is the 14th smallest prime number. The previous is forty-one, with which it comprises a twin prime, the next is forty-seven. Multiplied by -1, 43 is a Heegner number.

Forty-three is also:

• The atomic number of technetium.
• The designation of Interstate 43, a freeway in Wisconsin.
• The code for direct dial international phone calls to Austria.
• Part of the title of the historical British comedy show Hancock's Forty-Three Minutes, starring Tony Hancock.
• Part of the title of a historical short story by Hershel Finman titled Forty-Three Rubles, about Rabbi Zvi Elimelech Spira.